Round-trip YAML content of rubocop-migrations.yml

As we want to be able to round trip this YAML, but unfortunate the Ruby
Psych library does not preserve comments.

rubocop/rubocop-migrations.yml

-# Make sure to update the docs if this file moves. Docs URL: https://docs.gitlab.com/ee/development/migration_style_guide.html#when-to-use-the-helper-method
+---
 Migration/UpdateLargeTable:
   Enabled: true
   OverLimitTables: