[libc++] Bump minimum compiler requirements

Now that we've branched for the LLVM 14 release, our support window
moves to clang-13 and clang-14. Similarly, AppleClang 13 has been
released for some time now, so that should be the oldest compiler
we support, per our policy.

A possible follow-up would be to remove _LIBCPP_HAS_NO_CONCEPTS, since
I don't think we support any compiler that doesn't support concepts
anymore.

Differential Revision: https://reviews.llvm.org/D118831