Requiring up-to-date branches before merging

[thehowl]

With the github bot, we now show a red check to anyone who creates a PR from a non-modifiable branch, which allows us to always merge in master.

@Kouteki, do you think we can enable requiring up-to-date branches again?

@n0izn0iz has also proposed another interesting solution, merge queues:

The root problem is that we don't enforce that branches are up to date before merge. The full way to fix that is:

* don't allow to merge branches that are not up-to-date

* disallow any branch-protection bypass (for example "Allow maintainers to bypass branch protections" option)

* optional: enable merge queue to be able to merge PRs without having to maintain them up-to-date manually

I must warn that the merge queue has tradeoffs and requires to modify workflows files to also target the merge-queue as well as properly marking required jobs in the repo config

from this comment

We can also look to try out merge queues.

[n0izn0iz]

For reference, here's the merge queue overview