.github: Update compilers used for cross-build testing.

• Drop clang 9 build, and switch the remaining ubuntu build to the more modern clang 12.

• Update the label for the macos-latest builds which are now using clang 13 rather than clang 12.

Reviewed by: uqs
Pull Request: https://github.com/freebsd/freebsd-src/pull/599

(cherry picked from commit 49335eda0f5d80e9c86754b5ed95a285b49ef166)