Allow CodeReuse/ActiveRecord in finders

Description of the proposal

Our SQL finder classes seem like a natural place to allow AR methods, and indeed we disable this Rubocop rule in 141 locations across app/finders and ee/app/finders 😁 We still might want to keep it to force some consideration before use, but I can't really think of a good reason for that.

So I'm proposing this addition to .rubocop.yml (and removing the overrides):

CodeReuse/ActiveRecord:
  Enabled: true
  Exclude:
    - 'app/finders/**/*.rb'
    - 'ee/app/finders/**/*.rb'

• Mention the proposal in the next backend weekly call and the #backend channel to encourage contribution
• Proceed with the proposal once 50% of the maintainers have weighed in, and 80% of the votes are 👍
• Once approved, mention it again in the next backend weekly call and the #backend channel

/cc @gitlab-org/maintainers/rails-backend