Improve time of CI jobs and make more checks mandatory for PR merge

[yperbasis]

Can do something similar to #18582. Also get beefier/dedicated/several machines for the slowest "All tests / tests-windows (windows-2025)".

[VBulikov]

yperbasis added Imp1 as Importance

[anacrolix]

@lystopad, following from #19223 I propose:

• Move windows jobs back to GitHub runners where it makes sense, or bring the caching improvements over to Windows selfhosted.
• Cleanup CI gating #19909
• Enable merge queues. Use merge queues in CI #19673. This becomes easy with a gate job, it is adjusted in one change to accommodate both PRs and merge queue.

Independently:

• Find some way to accelerate the non-native jobs, if at all possible (assertoor mainly?), or just rely on merge queues for this one.