Draft: Add Rubocop: prefer `size` to `count`

What does this MR do and why?

• We already have a similar check that is only for HAML files where each is called: !28883 (merged)
• It should always be safe to use size instead of count because size calls count if the records are not already loaded.
• https://github.com/rails/rails/blob/94b5cd3a20edadd6f6b8cf0bdf1a4d4919df86cb/activerecord/lib/active_record/relation.rb#L210
• https://www.speedshop.co/2019/01/10/three-activerecord-mistakes.html

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.