Move Ubuntu CI from 20.04 to 22.04

[alilleybrinker]

GitHub is in the process of phasing out support for Ubuntu 20.04 in GitHub Actions. Their policy is that the two most recent versions of stable Ubuntu releases are available, and since Ubuntu 24.04 just became Generally Available, GitHub has now initiated their wind-down of support for it.

actions/runner-images#11101

For us, this means two things:

• We'll need to move our testing actions to use Ubuntu 22.04.
• We'll need to explore options for getting our release action, generated by dist, to use 22.04.

The first one is easy; we can introduce an experimental new action that runs on Ubuntu 22.04, run that in parallel as a non-PR-blocking step, and when we're confident, remove 20.04 and just use 22.04.

The second one is harder. The release action is generated by dist, and during CI runs dist validates that we haven't edited it and errors out if we have. This means we need dist to generate an action with 22.04 instead of 20.04. There may be an option to configure that, or they may already be working on it. I haven't checked yet.

We do want both the release and testing actions to be in sync, so we can feel confident we won't hit surprise issues during release CI because of a platform inconsistency.

[alilleybrinker]

Looks like the dist folks are aware and will likely bump to 22.04 soon. In the meantime there is a config item to override it: axodotdev/cargo-dist#1760

[alilleybrinker]

Here's the config for setting our own runner choices as well. Seems like something we probably want to take explicit control of anyway. For now I'd say it makes sense to do the following:

• Move Ubuntu builds to ubuntu-22.04
• Move Apple Silicon macOS builds to macos-14 (which uses Apple Silicon)

The second fix would also mean our CI tests and deploy action for Apple Silicon Macs would now match, both on macos-14.