Restore iOS testing on current Xcode.

[freakboy3742]

Restores the use of the current Xcode and iOS simulator.

As part of the update, removes the use of the deprecated, and soon to be removed macOS-12 runner (using macOS-13, which is the last remaining x86_64 runner), and switches to macOS-latest for all other uses (which will be ARM64 going forward).

I also tried adding a requires-python declaration to testbed, since Briefcase now supports that feature; but that doesn't work because of the requirement for 3.12 for Wayland testing. However, I've updated contribution docs to highlight the general requirement; when we update the testbed to use more a more recent Pillow, we can switch to 3.12 across the board.

Fixes #2476.
Fixes #2883.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

Still not working on the 20240415.6 image.

[randiaz95]

I think the flaw is the error: "This backend doesn't use keyboard shortcuts" maybe we should remove keyboard shortcuts tests for iOS?

[freakboy3742]

I think the flaw is the error: "This backend doesn't use keyboard shortcuts" maybe we should remove keyboard shortcuts tests for iOS?

I'm sorry? What has keyboard shortcuts got to do with this?

This is 100% due to a regression in the behaviour of GitHub Actions CI. The tests did pass, and still do pass, as long as you're using a GitHub Actions image that hasn't been affected by the regression.

[freakboy3742]

It looks like the issue with the iOS simulator on GitHub Actions has been resolved... but has introduced new testing problems caused by the new default iOS version being iOS 18.