[libc++] Stop running backdeployment CI on macOS 10.9

The oldest deployment target supported by Xcode 14 is macOS 10.13.
Trying to back-deploy to older targets runs into other issues in Clang,
so stop testing libc++ against unsupported deployment targets.

This patch doesn't attempt to clean up support for older deployment
targets from the code base -- this will be done in a follow-up patch.

Differential Revision: https://reviews.llvm.org/D155085