build: set minimum supported macOS to 10.12
Summary:
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. 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.Backport of core PR17550.
Test Plan:
Run the OSX Gitian build.
Run the binary on 10.12 and 10.15.
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D5514