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

7d7bf2ff4a 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 7d7bf2ff4a
  dongcarl:
    post-IRC-grilling-ACK 7d7bf2ff4a

Tree-SHA512: 30b0ed75e3b4df2d3f94db2091b7982c1ea7be8fb3a2732d8efd1a53ef1e492d7265a47e90c38e92f66e638d10e9400a6ecb56e5093688bb26d1621645b453e8

contrib/gitian-descriptors/gitian-osx.yml

@@ -37,7 +37,7 @@ script: |
   set -e -o pipefail
 
   WRAP_DIR=$HOME/wrapped
-  HOSTS="x86_64-apple-darwin14"
+  HOSTS="x86_64-apple-darwin16"
   CONFIGFLAGS="--enable-reduce-exports --disable-bench --disable-gui-tests GENISOIMAGE=$WRAP_DIR/genisoimage"
   FAKETIME_HOST_PROGS=""
   FAKETIME_PROGS="ar ranlib date dmg genisoimage"