Update minimum Xcode version enforced in the tool to 12.3 (released December 2020) so that later that version of clang can be used in the engine builder (the tool minimum needs to stay ahead of the builder for bitcode support).

Fortunately, the plugins repo is already on Xcode 13 and the devicelab is already running 12.3.

I told @csells we wouldn't bump to Xcode 13 until next year, and anyway that will involve more devicelab work
#91634, so just get to 12.3 for now.

This is necessary to support building the ARM macOS framework on the engine builder #84453, which needs the engine to build with a version of clang that can compile with the macOS Big Sur SDK.

Previous bump at #77025