Merge #19305: doc: add C++17 release note for 0.21.0

f1d21ef1c370898ae10569f2a8569bd98d71a981 doc: add C++17 release note for 0.21.0 (fanquake)

Pull request description:

  TLDR: Mention that the codebase is now compatible with C++17, and that the
  intention is to require C++17 starting with 0.22.0.

  Following some discussion with Cory/Carl, and in #16684, I think this is the next step in the C++17 migration.

  While #16684 mentions a gitian/Guix release with C++17, it's not yet clear how that would be done. Are we just going to pass `--enable-c++17` in gitian/Guix?. Are we changing our default in configure.ac?

  According to the [last comment](https://github.com/bitcoin/bitcoin/issues/16684#issuecomment-643778757) in #16684, we wouldn't be changing anything in depends:
  > No, everything (including depends) will stay at C++11.

  However I don't think we want to be mixing C++11 built dependencies, with a C++17 built bitcoind, if there is any potential for compatibility issues.

  Instead, I'd suggest we build the 0.21.0 release as C++11, and do a complete switch to C++17 for 0.22.0. Also, if we actually wanted to use C++17 in depends for 0.21.0, we couldn't without breaking C++11 compat (Qt). See below.

  Here is a potential timeline/TODOs for the migration:

  Potential Timeline
  * 17 / 6 / 2020 - Today
  * Some time prior to split-off:
      * Confirm that compiling with C++17 works.
      * Confirm that C++11 compatibility has not been broken.
  * 1 / 11 / 2020
      * [0.21.0 split off happens](https://github.com/bitcoin/bitcoin/issues/18947).
  * 2 / 11 / 2020
      * Merge an "incompatible with C++11" change into master.
      * Switch configure to use C++17 mode by default.
      * Update minimum compiler requirements. At least:
          * Clang 5: https://clang.llvm.org/cxx_status.html#cxx17
          * GCC 7: https://gcc.gnu.org/projects/cxx-status.html#cxx17
              * While GCC has some support from 5, it seems a more complete support landed in GCC 7.
              * https://gcc.gnu.org/gcc-7/changes.html#cxx
      * Switch depends packages to use C++17 where applicable.
      * Bump Qt from 5.9.x (no c++17 mode) to, likely, 5.15.x (LTS).
      * Drop support for macOS < 10.14.x
          * The c++ dylib shipped with macOS [doesn't support c++17, prior to macOS 10.14](https://github.com/bitcoin/bitcoin/issues/16684#issuecomment-643722538).
          * Building Qt 5.12 or 5.15 in C++17 mode will also require a minimum macOS deployment target of 10.14. https://codereview.qt-project.org/c/qt/qtbase/+/283832.
      * Begin merging PRs like #19183 and #19245.
          * I've left some comments in #19183 if the macOS runtime issue interests anyone.
  * 3 / 12 / 2020
      * 0.21.0 released.
      * Built as C++11.
      * Contains warning in release notes that compiling 0.22.0 will require C++17.
  * 3 / 6 / 2021
      * 0.22.0 released.
      * Full of C++17 code.

  One thing worth noting, is that we cannot bump our Qt to a newer LTS for 0.21.0, without breaking C++11 compatibility. Qt 5.12 is not compilable in C++11 mode, as the project has started using C++14 features throughout at least the macOS portions of it's codebase, and seemingly "forgotten" that the release is meant to be C++11 compatible.
  Upstream bug here: https://bugreports.qt.io/browse/QTBUG-77310.
  > Building Qt requires C+11, at a minimum, but in practice we use later features, usually under a feature define, or with a fallback of some kind. On platforms that support > C11, we've (apparently) not considered the fallback necessary, under the assumption C+14 is always available.

ACKs for top commit:
  MarcoFalke:
    ACK f1d21ef1c370898ae10569f2a8569bd98d71a981 can't hurt to give an advance warning
  Sjors:
    ACK f1d21ef1c370898ae10569f2a8569bd98d71a981
  laanwj:
    ACK f1d21ef1c370898ae10569f2a8569bd98d71a981
  theStack:
    ACK f1d21ef1c370898ae10569f2a8569bd98d71a981

Tree-SHA512: 706baceb07d9584783ba6e437cdf447531c20f586285b9797edc21f3adb1e9d386059d1c543c70eb298d0f8e555dafb6682a55d35c5836979fc12132e8ba06f5

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