meta: automatically update PRs that have auto merge enabled

[ephys]

This PR adds a replacement for the merge queue. Instead of using the merge queue, we can assign the automerge label to a PR to make the CI automatically merge the PR when ready, as well as update it if it becomes outdated.

Added the automerge label on this PR to test it.

[ephys]

Hmmm. The CI checks are not running and I don't know if that's because the PR was a draft or if it's because a bot added the merge commit

When a pull request is merged by this action, the merge will not trigger other GitHub workflows. Similarly, when another GitHub workflow creates a pull request, this action will not be triggered. This is because an action in a workflow run can't trigger a new workflow run. However, the workflow_run event is triggered as expected.

• https://github.com/pascalgn/automerge-action

I guess we'll see the next time something is merged

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Also I'm realizing that we don't need the bot to handle the merging into develop part. We can synchronize the automerge label with the "enable auto-merge" button, and have it only auto-update instead.

Keeping undrafted to test things, sorry for the notifications. I'll request a re-review when ready

[ephys]

OK, new approach! This new version does not merge the PR to the target branch anymore.

Instead, we'll use github's native auto-merge, and this new action will keep the PR updated so the automerge can happen. It will run every time a commit is added to main, and will merge main back into every PR that targets it and has auto-merge enabled