Use gitlab-styles which provides shared RuboCop config and cops

EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3817

What does this MR do?

This uses https://gitlab.com/gitlab-org/gitlab-styles which includes a shared RuboCop config and custom cops that can be reused by multiple projects.

Will allow to solve #37711 (closed).

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

We can still define custom cops as before, and regularly migrate them to the gem if this is more practical (or for testing/debugging purpose).

Why was this MR needed?

So that we can avoid duplicating our RuboCop config and custop cops.

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #37711 (closed).