fix(CI): Reduce test load & speed up tests

[sentrivana]

• don't run every integration test suite separately; group them and run the groups in serial
• reduce the number of Python versions we test against
  • always test against lowest and highest supported
  • always test against 3.11 and 3.12 if supported
  • if it makes sense, pick some versions in between
• remove the retry action; we can apply it selectively later on to test suites that often fail; not every single one has to be rerun automatically

Once this has been reviewed it'll need some repo settings changes (marking different jobs as required). Right now it's complaining about "expected checks" which refers to the old "All <framework> tests passed or skipped" checks that are now obsolete.

[antonpirker]

great change!