Enable running tests & release artifacts on merge queue.

[clokep]

This is a step to enabling merge queues on Synapse. We might be able to slim down on PRs a little bit, maybe:

• Run portdb on once?
• Do not run sytest with and without asyncio?
• Do not build wheels / debs on PRs?

This only adds the merge queue for tests.yml & release-artifacts.yml; the other things which run for PRs are:

• docs-pr.yml: We build a preview of docs for PRs and then build them when we push to develop, I don't think we need to do it here too. (This check isn't part of our CI checks just a convenience.)
• dependabot_changelog.yml: These run when dependabot pushes PRs?
• poetry_lockfile.yaml: If two PRs modify the lockfile I don't see how this check could fail (it checks to ensure all deps have sdists -- if this is true for each PR individually it must be true for the combined?)

Not sure about others. My hope is to enable the merge queue on develop with the defaults (except where noted)

• Merge method: squash
• Build concurrency: 5
• Merge limits: Min of 1 after 5 minutes, max of 5
• Only merge non-failing pull requests: OFF (THIS IS NOT THE DEFAULT)
• Status check timeout: 60 minutes

[clokep]

I've enabled this for the develop branch only. We might want to enable for release-v* also, but we can look at that in the future if develop goes well.

[clokep]

We ended up disabling the merge queue:

• If a test flakes on it then you can wait ~1 hour for the merge to then fail, which isn't fun.
• The merge queue doesn't let you customize commit messages.
• We don't feel like we've been seeing the issue that this solves recently (conflicts when merging multiple PRs).