This change is necessary before enabling merge queues. The `merge_group`
event is needed to trigger the GitHub Actions workflow when a pull
request is added to a merge queue.
The merge queue provides the same benefits as the Require branches to be
up to date before merging branch protection but does not require a pull
request author to update their pull request branch and wait for status
checks to finish before trying to merge. More info on
https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/configuring-pull-request-merges/managing-a-merge-queue.
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