Add a cop that disallows the use of `execute.find_by` on a Finder

Instead we should use Finder#find_by defined in FinderMethods, only available after gitlab-ee!3709 is merged.

The following discussion from gitlab-ee!3709 should be addressed:

• @reprazent started a discussion: (+2 comments)

  Add a cop that disallows finders using the finder.execute.find(_by) pattern. Instead we should use finder.find as we have in IssuableFinder#find_by

/cc @DouweM