Enable rubocop for db/**/* and ee/db/**/*

What does this MR do?

Enable rubocop for db/**/* and ee/db/**/*

Are there points in the code the reviewer needs to double check?

I don't intend to change any existing migrations.

EE at: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6654

Why was this MR needed?

It's unfortunately we're not using those cops for so long...

Does this MR meet the acceptance criteria?

• Conforms to the code review guidelines
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist

What are the relevant issue numbers?

Closes #49582 (closed)