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