Make a Cop to ban `attributes_for` in migration specs
In #299382 (closed) some migration specs are using attributes_for
which will use FactoryBot which will use models so they're sensitive to model changes, and we're not running migration specs due to model changes, and we don't want to keep rewriting the migration specs because we changed the model.
This is an anti-pattern: https://docs.gitlab.com/ee/development/testing_guide/testing_migrations_guide.html#table
FactoryBot should not be used to create data for migration specs because it relies on application code which can change after the migration has run, and cause the test to fail.
We need to rewrite those specs, and add a cop to ban the use of this.