Move to Merge Trains for merging branches

I had planned to migrate ghc/ghc> from @marge-bot to Merge Trains. Unfortunately, while the feature has been improving steadily in recent releases, it turns out it's still not usable for our purposes. Specifically there are two issues:

• gitlab&3278: Allow fork pipelines to run as the parent project, allowing forks to work
• gitlab#207187: The SHA tested in CI is not the same as the SHA that ends up merged to the branch due to use of a temporary commit during CI.
• gitlab&4911: Fast-forward merge support for merge trains, including automatic rebasing of queued merges.
• gitlab&11650: Fast-forward Merge Train Complete Feature Maturity