Add Rubocop and Lefthook

Context

I wanted to work on gitlab-org/gitlab#450374 (closed), and while setting up my environment, my editor kept bothering me about Rubocop that could not be found 👮.

What's in this MR?

• [Refactor] Add gitlab-styles and lefthook
• [Refactor] Fix Rubocop offenses
• [BREAKING CHANGE] Drop support for Ruby < 3.0