Create cop to enforce exists? over any? and present? in ActiveRecord relations

Description of the proposal

This is the result of executing any?, exists? and present?:

User.where.not(preferred_language: 'en').present? => SELECT "users".* FROM "users" WHERE ("users"."preferred_language" != 'en') # Existence retrieved from the loaded records' array
User.where.not(preferred_language: 'en').any? => SELECT COUNT(*) FROM "users" WHERE ("users"."preferred_language" = 'en')
User.where.not(preferred_language: 'en').exists? => SELECT  1 AS one FROM "users" WHERE ("users"."preferred_language" != 'en') LIMIT 1

As you can see, exists? is faster because the query stops when it gets the first result.

# GOOD
User.where(preferred_language: 'en').exists?
User.exists?(preferred_language: 'en') # Even better

# BAD
User.where(preferred_language: 'en').any?
User.where(preferred_language: 'en').present?

• https://www.ombulabs.com/blog/benchmark/performance/rails/present-vs-any-vs-exists.html

• https://github.com/rubocop-hq/rails-style-guide/issues/232

• https://www.cybertec-postgresql.com/en/count-made-fast/

• 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 /cc @abrandl