[runtimes] Post-branch tasks for the LLVM 23 release (#176007)

This performs most of the post-branch tasks to start working on the LLVM
23 release. Things that are left to do:
- Update the unicode version
- Update CI versions and supported compiler versions

NOKEYCHECK=True
GitOrigin-RevId: ec983ad611fa15890fdcf5c3066e938b5b154459