Merge #17550: build: set minimum supported macOS to 10.12

7d7bf2ff4a67c3953b0d660ca88d11313cb4d71d build: set minimum supported macOS to 10.12 (fanquake)

Pull request description:

  Extracted from #16392 as this doesn't need to wait for the other build changes.

  Reasoning:
  * `10.10` has been unsupported since July 2017 (~3 years at `v0.20.0` release)
  * `10.11` has been unsupported since July 2018  (~2 years at `v0.20.0` release)
  * macOS users are consistent at upgrading to new releases.
  * Qt 5.12 LTS only supports [macOS > 10.12](https://doc-snapshots.qt.io/qt5-5.12/supported-platforms.html). As long as we're supporting macOS < 10.12 we would not be able to bump Qt in depends to 5.12 for the `v0.20.0` release.

  Once we drop support for 10.12 and start using the 10.15 SDK there are some other follow ups:
  * Enabling support for [`thread_local`](https://github.com/bitcoin/bitcoin/blob/master/configure.ac#L901).
  * Removing some of the macOS notification code
  * Removing macOS [startup item code](https://github.com/bitcoin/bitcoin/blob/master/src/qt/guiutil.cpp#L695).

  There was also some related discussion in the `#bitcoin-builds` channel yesterday arvo.

ACKs for top commit:
  laanwj:
    ACK 7d7bf2ff4a67c3953b0d660ca88d11313cb4d71d
  dongcarl:
    post-IRC-grilling-ACK 7d7bf2ff4a67c3953b0d660ca88d11313cb4d71d

Tree-SHA512: 30b0ed75e3b4df2d3f94db2091b7982c1ea7be8fb3a2732d8efd1a53ef1e492d7265a47e90c38e92f66e638d10e9400a6ecb56e5093688bb26d1621645b453e8

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