Meta: Consider introducing GitHub Merge Queue to the repo

[rami3l]

Maybe it's time to introduce a "conclusion" CI action and a merge queue. We already have automerge=true configured in renovate, but since there's no "conclusion" configured in our branch protection settings, auto-merging will not work. OTOH once that's been added, renovate will automatically put semver-compatible updates into the merge queue (cf. renovatebot/renovate#20569).

Originally posted by @rami3l in #3565 (comment)

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Tasks

• Merge ci: streamline CI actions to prepare for merge queue introduction #3582.
• Merge feat(ci): enable the merge_group trigger #3633.
• Tell the mods to enable GHMQ and to point the branch protection rules to the newly-added conclusion job.

[djc]

Sounds good to me!

[rami3l]

@djc For the record, an example of the conclusion step can be found in PyO3, however this will probably require moving all CI workflows into a single file (which should be done with a refactoring of ci/actions-templates/gen-workflows.sh), so this might take a while.

[rami3l]

New concern raised in #3582 (comment).

Update: Addressed in #3627.

[rami3l]

GHMQ is now online with "rebase and merge" as the default action.

Thanks a lot for your help and support, @jdno and @djc!