build: use macOS 11 SDK (Xcode 12.2)

Summary:
This is one of the steps needed to support building for Apple ARM when
cross-compiling.

This is a partial backport of core#21851
https://github.com/bitcoin/bitcoin/pull/21851/commits/446e73cc0bb7cd628039eaf9b1bcc93db23b924f
doc improvements from core#24241

Test Plan: gitian-osx

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14015