After a recent Github Actions runner policy change [1], the version of
Xcode included in the macos-14 image went from Xcode 16 to Xcode 15,
breaking our build bots.
This moves the bots to the macos 15 (public preview) image, which
contains Xcode 16.
Also, adjust an UNSUPPORTED annotation that was incorrectly targeting
macos 13.7 when it should have been targeting a version of AppleClang.
[1]: https://github.com/actions/runner-images/issues/10703
