Migration specs still run in `rspec unit` jobs

In !20845 (diffs), I forgot to remove migrations from the unit pattern. 🤦

That means migrations specs still run in unit jobs. Fortunately, the fix to drop/recreate the DB when there are too many columns (introduced in !20845 (diffs) as well) is working fine.

I noticed that while looking at this Periscope chart: https://app.periscopedata.com/app/gitlab/496118/Engineering-Productivity-Sandbox?widget=6914189&udv=785399