ci: tracking: migrate to github merge trains

[kaspar030]

This issue tracks the migration to github merge trains. Will update as we go along.

• [x] make murdock build merge train branches #20052
• [x] create branch merge_queue_migration, try to get the workflows to fly there
  • [x] enable merge queues in RIOT repo (for merge_queue_migration)
  • [x] fix "slow static-tests" #20061
  • [x] drop "limit build" commit
• [ ] enable merge queues for master && PR merge_queue_migration to master #20063
• [x] update branch protections
• [ ] stop murdock builds of cancelled merge_group runs

follow up:

• [ ] solve merge_group for release branches
• [ ] remove bors configuration

[MrKevinWeiss]

Note: When removing a PR from the mq, it appears that murdock still tries to finish building instead of cancelling. Maybe this is due to not listening to merge_groups_mirgration branch and only master. Maybe we need to listen to the gh-readonly-queue/* branch closing. Somehow we need to cancel the murdock job when removing PRs from the queue.

[MrKevinWeiss]

It seems force pushing something to a PR that is already in a mq does not remove the mq branch from the queue

[MrKevinWeiss]

We still need to verify that the branch protection rules are set so that a merge train is possible and what happens to the other PRs when one fails.

[maribu]

I think this can be closed, can't it?