Use GitHub-hosted macos-12 in CI, not deprecated macos-11

[huonw]

GitHub Actions has deprecated their macos-11 hosted runners, and removing them at the end of the month. This thus bumps to macos-12, the oldest supported version.

https://github.blog/changelog/2024-05-20-actions-upcoming-changes-to-github-hosted-macos-runners/

This cherry-picks to all active branches to ensure their CI continues to work.

Fixes #20938

[benjyw]

We were running the MacOS 11 x86_64 CI shards on a self-hosted runner (IIRC, because the GH hosted ones were horrifically unreliable).

But looks like these ran on hosted runners?

[huonw]

It looks like for macOS x86-64:

• 11: github-hosted (changed here)
• 10.15: self-hosted (not changed)

(Not directly related to this PR, but I've assumed that we have the 10.15 self-hosted runner around to be able to do releases that are still compatible with that older version.)

[benjyw]

Hmm, yes, looks like there were/are 3 macos platforms in play.

[huonw]

Four if we go with #20997! 😅

[WorkerPants]

I tried to automatically cherry-pick this change back to each relevant milestone, so that it is available in those older releases of Pants.

❌ 2.20.x

I was unable to cherry-pick this PR to 2.20.x, likely due to merge-conflicts.

Steps to Cherry-Pick locally

To resolve:

1. (Ensure your git working directory is clean)
2. Run the following script to reproduce the merge-conflicts:

   git fetch https://github.com/pantsbuild/pants main \
     && git fetch https://github.com/pantsbuild/pants 2.20.x \
     && git checkout -b cherry-pick-21003-to-2.20.x FETCH_HEAD \
     && git cherry-pick 5fe452588a9a5b12b566fa3428ef7fc26f7660ba

3. Fix the merge conflicts and commit the changes
4. Run build-support/cherry_pick/make_pr.sh "21003" "2.20.x"

Please note that I cannot re-run CI if a job fails. Please work with your PR approver(s) to re-run CI if necessary.

❌ 2.21.x

I was unable to cherry-pick this PR to 2.21.x, likely due to merge-conflicts.

Steps to Cherry-Pick locally

To resolve:

1. (Ensure your git working directory is clean)
2. Run the following script to reproduce the merge-conflicts:

   git fetch https://github.com/pantsbuild/pants main \
     && git fetch https://github.com/pantsbuild/pants 2.21.x \
     && git checkout -b cherry-pick-21003-to-2.21.x FETCH_HEAD \
     && git cherry-pick 5fe452588a9a5b12b566fa3428ef7fc26f7660ba

3. Fix the merge conflicts and commit the changes
4. Run build-support/cherry_pick/make_pr.sh "21003" "2.21.x"

Please note that I cannot re-run CI if a job fails. Please work with your PR approver(s) to re-run CI if necessary.

---

When you're done manually cherry-picking, please remove the needs-cherrypick label on this PR.

Thanks again for your contributions!

🤖 Beep Boop here's my run link