[llvm] [libc++] Bump OS version for macOS backdeployment CI jobs (PR #131883)

[Mark de Wever via llvm-commits]

https://github.com/mordante requested changes to this pull request.

Thanks for working on this! Removing old apple-clang versions will help to unblock some cleanup patches.

I think we need one more change.

https://github.com/llvm/llvm-project/pull/131883