a43b8e9555 build: set OSX_MIN_VERSION to 10.15 (fanquake)
Pull request description:
Taken out of #20744, as splitting up some of the build changes was mentioned [here](https://github.com/bitcoin/bitcoin/pull/22937#discussion_r707303172).
This is required to use `std::filesystem` on macOS, as support for it only landed in the libc++.dylib shipped with 10.15. So if we want to move to using `std::filesystem` for `23.0`, this bump is required.
See also: https://developer.apple.com/documentation/xcode-release-notes/xcode-11-release-notes
> Clang now supports the C++17 \<filesystem\> library for iOS 13, macOS 10.15, watchOS 6, and tvOS 13.
macOS 10.15 was released in October 2019. macOS OS's seem to have a life of about 3 years, so it's possible that 10.14 will become officially unsupported by the end of 2021 and prior to the release of 23.0.
Guix builds:
```bash
bash-5.1# find guix-build-$(git rev-parse --short=12 HEAD)/output/ -type f -print0 | env LC_ALL=C sort -z | xargs -r0 sha256sum
abc8b749be65f1339dcdf44bd1ed6ade2533b8e3b5030ad1dde0ae0cede78136 guix-build-a43b8e955558/output/dist-archive/bitcoin-a43b8e955558.tar.gz
1edcc301eb4c02f3baa379beb8d4c78e661abc24a293813bc9d900cf7255b790 guix-build-a43b8e955558/output/x86_64-apple-darwin19/SHA256SUMS.part
e9dbb5594a664519da778dde9ed861c3f0f631525672e17a67eeda599f16ff44 guix-build-a43b8e955558/output/x86_64-apple-darwin19/bitcoin-a43b8e955558-osx-unsigned.dmg
11b23a17c630dddc7594c25625eea3de42db50f355733b9ce9ade2d8eba3a8f3 guix-build-a43b8e955558/output/x86_64-apple-darwin19/bitcoin-a43b8e955558-osx-unsigned.tar.gz
257ba64a327927f94d9aa0a68da3a2695cf880b3ed1a0113c5a966dcc426eb5e guix-build-a43b8e955558/output/x86_64-apple-darwin19/bitcoin-a43b8e955558-osx64.tar.gz
```
ACKs for top commit:
hebasto:
ACK a43b8e9555
jarolrod:
ACK a43b8e9
Tree-SHA512: 9ac77be7cb56c068578860a3b2b8b7487c9e18b71b14aedd77a9c663f5d4bb19756d551770c02ddd12f1797beea5757b261588e7b67fb53509bb998ee8022369
8f022a59b8