[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
19 files changed