[gn] bump deployment target to macOS 13 (#227124)
macOS 13 is four years old by now.
This has the effect that lld starts defaulting to chained fixups with
this. Chained fixups reduces `clang --version` from 4 ms to 3.2 ms. (Not
that it matters.)
(Without #227120, chained fixups reduce `clang --version` from 12.5 ms
to 10.9 ms.)
No behavior change.
