doc: archive 27.0 release notes

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+Bitcoin Core version 27.0 is now available from:
+
+  <https://bitcoincore.org/bin/bitcoin-core-27.0/>
+
+This release includes new features, various bug fixes and performance
+improvements, as well as updated translations.
+
+Please report bugs using the issue tracker at GitHub:
+
+  <https://github.com/bitcoin/bitcoin/issues>
+
+To receive security and update notifications, please subscribe to:
+
+  <https://bitcoincore.org/en/list/announcements/join/>
+
+How to Upgrade
+==============
+
+If you are running an older version, shut it down. Wait until it has completely
+shut down (which might take a few minutes in some cases), then run the
+installer (on Windows) or just copy over `/Applications/Bitcoin-Qt` (on macOS)
+or `bitcoind`/`bitcoin-qt` (on Linux).
+
+Upgrading directly from a version of Bitcoin Core that has reached its EOL is
+possible, but it might take some time if the data directory needs to be migrated. Old
+wallet versions of Bitcoin Core are generally supported.
+
+Compatibility
+==============
+
+Bitcoin Core is supported and extensively tested on operating systems
+using the Linux Kernel 3.17+, macOS 11.0+, and Windows 7 and newer. Bitcoin
+Core should also work on most other Unix-like systems but is not as
+frequently tested on them. It is not recommended to use Bitcoin Core on
+unsupported systems.
+
+Notable changes
+===============
+
+libbitcoinconsensus
+-------------------
+
+- libbitcoinconsensus is deprecated and will be removed for v28. This library has
+  existed for nearly 10 years with very little known uptake or impact. It has
+  become a maintenance burden.
+
+  The underlying functionality does not change between versions, so any users of
+  the library can continue to use the final release indefinitely, with the
+  understanding that Taproot is its final consensus update.
+
+  In the future, libbitcoinkernel will provide a much more useful API that is
+  aware of the UTXO set, and therefore be able to fully validate transactions and
+  blocks. (#29189)
+
+mempool.dat compatibility
+-------------------------
+
+- The `mempool.dat` file created by -persistmempool or the savemempool RPC will
+  be written in a new format. This new format includes the XOR'ing of transaction
+  contents to mitigate issues where external programs (such as anti-virus) attempt
+  to interpret and potentially modify the file.
+
+  This new format can not be read by previous software releases. To allow for a
+  downgrade, a temporary setting `-persistmempoolv1` has been added to fall back
+  to the legacy format. (#28207)
+
+P2P and network changes
+-----------------------
+
+- BIP324 v2 transport is now enabled by default. It remains possible to disable v2
+  by running with `-v2transport=0`. (#29347)
+- Manual connection options (`-connect`, `-addnode` and `-seednode`) will
+  now follow `-v2transport` to connect with v2 by default. They will retry with
+  v1 on failure. (#29058)
+
+- Network-adjusted time has been removed from consensus code. It is replaced
+  with (unadjusted) system time. The warning for a large median time offset
+  (70 minutes or more) is kept. This removes the implicit security assumption of
+  requiring an honest majority of outbound peers, and increases the importance
+  of the node operator ensuring their system time is (and stays) correct to not
+  fall out of consensus with the network. (#28956)
+
+Mempool Policy Changes
+----------------------
+
+- Opt-in Topologically Restricted Until Confirmation (TRUC) Transactions policy
+  (aka v3 transaction policy) is available for use on test networks when
+  `-acceptnonstdtxn=1` is set. By setting the transaction version number to 3, TRUC transactions
+  request the application of limits on spending of their unconfirmed outputs. These
+  restrictions simplify the assessment of incentive compatibility of accepting or
+  replacing TRUC transactions, thus ensuring any replacements are more profitable for
+  the node and making fee-bumping more reliable. TRUC transactions are currently
+  nonstandard and can only be used on test networks where the standardness rules are
+  relaxed or disabled (e.g. with `-acceptnonstdtxn=1`). (#28948)
+
+External Signing
+----------------
+
+- Support for external signing on Windows has been disabled. It will be re-enabled
+  once the underlying dependency (Boost Process), has been replaced with a different
+  library. (#28967)
+
+Updated RPCs
+------------
+
+- The addnode RPC now follows the `-v2transport` option (now on by default, see above) for making connections.
+  It remains possible to specify the transport type manually with the v2transport argument of addnode. (#29239)
+
+Build System
+------------
+
+- A C++20 capable compiler is now required to build Bitcoin Core. (#28349)
+- MacOS releases are configured to use the hardened runtime libraries (#29127)
+
+Wallet
+------
+
+- The CoinGrinder coin selection algorithm has been introduced to mitigate unnecessary
+  large input sets and lower transaction costs at high feerates. CoinGrinder
+  searches for the input set with minimal weight. Solutions found by
+  CoinGrinder will produce a change output. CoinGrinder is only active at
+  elevated feerates (default: 30+ sat/vB, based on `-consolidatefeerate`×3). (#27877)
+- The Branch And Bound coin selection algorithm will be disabled when the subtract fee
+  from outputs feature is used. (#28994)
+- If the birth time of a descriptor is detected to be later than the first transaction
+  involving that descriptor, the birth time will be reset to the earlier time. (#28920)
+
+Low-level changes
+=================
+
+Pruning
+-------
+
+- When pruning during initial block download, more blocks will be pruned at each
+  flush in order to speed up the syncing of such nodes. (#20827)
+
+Init
+----
+
+- Various fixes to prevent issues where subsequent instances of Bitcoin Core would
+  result in deletion of files in use by an existing instance. (#28784, #28946)
+- Improved handling of empty `settings.json` files. (#29144)
+
+Credits
+=======
+
+Thanks to everyone who directly contributed to this release:
+
+- 22388o⚡️
+- Aaron Clauson
+- Amiti Uttarwar
+- Andrew Toth
+- Anthony Towns
+- Antoine Poinsot
+- Ava Chow
+- Brandon Odiwuor
+- brunoerg
+- Chris Stewart
+- Cory Fields
+- dergoegge
+- djschnei21
+- Fabian Jahr
+- fanquake
+- furszy
+- Gloria Zhao
+- Greg Sanders
+- Hennadii Stepanov
+- Hernan Marino
+- iamcarlos94
+- ismaelsadeeq
+- Jameson Lopp
+- Jesse Barton
+- John Moffett
+- Jon Atack
+- josibake
+- jrakibi
+- Justin Dhillon
+- Kashif Smith
+- kevkevin
+- Kristaps Kaupe
+- L0la L33tz
+- Luke Dashjr
+- Lőrinc
+- marco
+- MarcoFalke
+- Mark Friedenbach
+- Marnix
+- Martin Leitner-Ankerl
+- Martin Zumsande
+- Max Edwards
+- Murch
+- muxator
+- naiyoma
+- Nikodemas Tuckus
+- ns-xvrn
+- pablomartin4btc
+- Peter Todd
+- Pieter Wuille
+- Richard Myers
+- Roman Zeyde
+- Russell Yanofsky
+- Ryan Ofsky
+- Sebastian Falbesoner
+- Sergi Delgado Segura
+- Sjors Provoost
+- stickies-v
+- stratospher
+- Supachai Kheawjuy
+- TheCharlatan
+- UdjinM6
+- Vasil Dimov
+- w0xlt
+- willcl-ark
+
+
+As well as to everyone that helped with translations on
+[Transifex](https://www.transifex.com/bitcoin/bitcoin/).