[libc++] Bump AppleClang compiler requirement Per our policy, the latest released AppleClang has been 14 for a while, so libc++ is removing support for AppleClang 13. Our CI bots have been moved to AppleClang 14 a few weeks ago. Differential Revision: https://reviews.llvm.org/D138685 GitOrigin-RevId: 396fbe264f5a43a57241043f66c1df3c526bc1ca