RFR: Upgrade GitHub Action Runner image for macOS to macos-12

[Johnny Lim]

This PR upgrades GitHub Action Runner image for macOS to `macos-12` as `macos-11` seems to have been removed.

See https://github.com/actions/runner-images?tab=readme-ov-file#available-images and https://github.com/actions/runner-images/issues/9255

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Commit messages:
 - Upgrade GitHub Action Runner image for macOS to macos-12

Changes: https://git.openjdk.org/jcstress/pull/152/files
  Webrev: https://webrevs.openjdk.org/?repo=jcstress&pr=152&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jcstress/pull/152.diff
  Fetch: git fetch https://git.openjdk.org/jcstress.git pull/152/head:pull/152

PR: https://git.openjdk.org/jcstress/pull/152