Libc++'s policy is to support only the latest released Xcode, which is
Xcode 16.x. We did update our CI jobs to Xcode 16.x, but we forgot to
update the documentation, which still mentioned Xcode 15. This patch
updates the documentation and cleans up outdated mentions of
apple-clang-15 in the test suite.
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
