fix: don't build universal2 as a default on macOS arm64 runners

[mayeut]

Per discussion in #1204, only build arm64 on arm64 macOS runners.

Fixes #1204
Relates to #1191

[joerick]

Just pushed a few docs updates. The only other thing I was wondering about was removing this line:

cibuildwheel/test/utils.py

Line 203 in 03c8f5b

f'macosx_{macosx_deployment_target.replace(".", "_")}_universal2',

...because we no longer expect it to be built by default. Or perhaps we need another parameter on expected_wheels() to enable it when we're specifying ARCH=universal2 in the tests.

[joerick]

I pushed a commit that should fix the expected_wheels thing. Of course, we'll only know once we try it on an arm64 CI, which should be coming in #1191 !