CI: macos-13 runner image deprecated, drops macOS x86-64 support

[rgommers]

We'll need to stop using this image in CI very soon, because GHA will start doing its normal deprecation thing with intermittently making the runners unavailable to alert users to the deprecation.

I posted on the mailing list about potentially dropping support for x86-64 wheels on PyPI.

[andyfaff]

That seems ok to me. Would clear up a lot of things.

[rgommers]

The outcome of the mailing list discussion (so far at least) is to try and build x86-64 wheels on macOS arm64 GHA runners under Rosetta. If that works, it'd be nice to keep supporting this platform for another year or so.