[llvm] [Github][libc++] Prefer ubuntu-24.04 over ubuntu-latest (PR #129949)

[Mark de Wever via llvm-commits]

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

Thanks for catching this! Did you manually verify this or did you use a script?

LGTM!

https://github.com/llvm/llvm-project/pull/129949