Merge #20419: build: set minimum supported macOS to 10.14

a52ecc936a build: set minimum supported macOS to 10.14 (fanquake)

Pull request description:

  This is a requirement for C++17 support. See my comments [here](https://github.com/bitcoin/bitcoin/issues/16684#issuecomment-643722538):

  > You cannot use std::get with std::variant on macOS < 10.14, because Apples libc++ doesn't support the std::bad_variant_access exception. [Relevant comment](https://github.com/bitcoin/bitcoin/pull/19183#discussion_r439794318) in #19183.

  > While we could work around this in our own code, using std::get_if, this would still be a problem for 3rd-party dependencies.

  > I've been testing Qt 5.15LTS (we'll have to enable C++17 in qt, and may upgrade to a newer version at the same time), and you can't enable -std c++17, while targeting a macOS deployment version < 10.14, configuring will fail. They are making use of std::get with std::variant throughout their cocoa code.

  We would have to had to have bumped to at least 10.13 in any case, as Qt 5.15 (#19716) [requires 10.13+](https://doc.qt.io/qt-5/supported-platforms.html).

ACKs for top commit:
  hebasto:
    ACK a52ecc936a, I have reviewed the code and it looks OK, I agree it can be merged.

Tree-SHA512: f669b2fc777aeea1e9afdbbc7bd9afe3997418211db6ba53c934cae0e62a9b999603da539518c229f34961d275c9e2f315c7b022cf5fb97bd201a69c85d470cc

depends/README.md

@@ -25,7 +25,7 @@ Common `host-platform-triplets` for cross compilation are:
 - `i686-pc-linux-gnu` for Linux 32 bit
 - `x86_64-pc-linux-gnu` for x86 Linux
 - `x86_64-w64-mingw32` for Win64
-- `x86_64-apple-darwin16` for macOS
+- `x86_64-apple-darwin18` for macOS
 - `arm-linux-gnueabihf` for Linux ARM 32 bit
 - `aarch64-linux-gnu` for Linux ARM 64 bit
 - `powerpc64-linux-gnu` for Linux POWER 64-bit (big endian)

depends/hosts/darwin.mk

@@ -1,4 +1,4 @@
-OSX_MIN_VERSION=10.12
+OSX_MIN_VERSION=10.14
 OSX_SDK_VERSION=10.15.1
 XCODE_VERSION=11.3.1
 XCODE_BUILD_ID=11C505