ci: improve action performance

[brendt]

For this one I used Claude's help instead of Aidan's :p

[brendt]

Ok the integration test run isn't shown, so I'll link it manually: https://github.com/tempestphp/tempest-framework/actions/runs/22844937934

My reasoning with these changes is:

• Concurrency just makes a lot of sense
• Reducing the matrix: I know in theory we're missing out on specific cases now, but I want to give this a try anyway. I don't think there ever was a PR where one specific matrix-entry failed; if something fails, it's always a bunch of entries failing together. So my suggestion is to try this out, and see whether we actually run into issues or not. If we do, we can always revert.