Remove AddForeignKeyConstraintToActiveStorageAttachmentsForBlobId m…

…igration

In rails#33419, we added this migration to
properly upgrade Active Storage from 5.2 to 6.0

On Rails 6.1 `rails app:update` shouldn't add this migration to users' app.

Note that, I've left implementation that makes `rails app:update` to generate
migrations for users' app that are in `activestorage/db/update_migrate/`
because we are likely to need it e.g.: rails#34935, rails#36835.