Move CI to "ubuntu-latest"

[JelleZijlstra]

GitHub is decommissioning Ubuntu 20.04. I wouldn't expect our tests to
have a lot of OS version dependencies, so let's try just running
ubuntu-latest everywhere.

[JelleZijlstra]

Oh, but the latest runner doesn't have some of the versions we want... I wonder if we should switch to uv to get Python binaries.

[JelleZijlstra]

uv's Python is also problematic:

• 3.8.0 fails mysteriously with Error: The process '/opt/hostedtoolcache/uv/0.6.11/x86_64/uv' failed with exit code 1 and no details. (I don't blame them, 3.8 is long out of support.)
• Most versions don't come with the CPython test suite pre-installed, which means we can't run CPython tests. Not an unreasonable decision for uv but seems like a deal breaker for us. We could clone the CPython repo ourselves and get the test suite that way, but that's more work.

[JelleZijlstra]

Now we're down to:

• 3.8.0, 3.9.0, and 3.10.0 aren't available. Should probably drop 3.8.0 and for 3.9 and 3.10 use the earliest version that GH Actions does have.
• SQLAlchemy tests segfault for some reason

I don't have more time for this right now, but if anyone reading wants to work on a way to get CI back to a reliable state, feel free!

[brianschubert]

if anyone reading wants to work on a way to get CI back to a reliable state, feel free!

Put up #570 :)