Modernize GitHub Actions test runner

[Repiteo]

Cleans up and consolidates the workflows for running tests. Now everything is handled via matrix in one file. Tests across 5 GitHub-hosted runners (macos-15, ubuntu-22.04, ubuntu-24.04, windows-2022, windows-2025), each using the min/max supported Python versions (3.9-3.14). As versions below 3.9 aren't included, this could wait until #4827 is merged

Contributor Checklist:

• I have created a new test or updated the unit tests to cover the new/changed functionality.
• I have updated CHANGES.txt and RELEASE.txt (and read the README.rst).
• I have updated the appropriate documentation

[bdbaddog]

Why did you remove appveyor? It's still being used.

Looks like you took info from a bunch of files, deteled them and stuck all that logic in one big file.
I'm not sure that's an improvement...

Perhaps a bit more explanation than what's in the PR above would be helpful as to the why you made these changes?

[mwichmann]

As usual when tweaking the CI, which happens one or more times a year, it probably makes sense to think about what we need to accomplish: operating systems, python versions, architectures, build frequency. The build matrix could be massive but that's obviously impractical: Linux/Windows/Mac; x86/arm64; 3.7 (or 3.9) - 3.14 and devel; trigger all on every commit to a PR and to main or be more selective with some jobs, etc. Oldest-and-newest Python doesn't seem a terrible combo to me.