GHA: Bump macOS images to 13 and 14

[kinke]

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[kinke]

The macos-14 GHA runners are ARM64 ones, so using Rosetta emulation all the way. And that seems to be uber-slow for the DMD testsuite (maybe because spawning a lot of compile & link processes?).

It might be worth a try to use an LDC host compiler for that job, and building/using an ARM64 DMD (which obviously still generates x86_64 code). Edit: Oh, no dmd.root.longdouble.longdouble_soft support when building DMD for a non-x86 architecture.