Add merge automation workflow

We currently have a merge script at scripts/ci/merges/do_merge.py. It should be getting to a point where it doesn't need to be run manually. We need to add some more merge driver support first. Once we're at that point, let's create a github workflow that can be manually triggered to run the merge.

Additional things the merge workflow should do (mostly taken from docs/release/RELEASE.md merge steps):

main->beta

1. Send a message to the #tb-mobile-dev:mozilla.org matrix channel to let them know the merge is starting
2. Run merge
3. Open a new milestone for the new version and close the oldest milestone (there should always be 3 open milestones - automation that adds tasks to milestones depends on this) - this needs to be done before version bump in main
4. Submit a pull request with the merge results. We still need a human to look at the results.
5. Submit a pull request that increments the version in main

beta->release

• Same as above but no changes to milestones and no pull request to increment version

Start with scripts/ci/merges/do_merge.py and the merge steps in docs/release/RELEASE.md, then inspect the existing GitHub workflow conventions. The workflow should be manually triggerable, support main-to-beta and beta-to-release merges, notify the Matrix channel, handle the specified milestone changes, and open the required pull requests for human review.