Enable rubocop cop in participants service again.
Summary
While working on gitlab-foss#53011 (moved) some changes were made in participants_service.rb class. Those changes required a disabling rubocop cop regarding the reusing of ActiveRecord methods.
Improvements
We need to refactor code so we don't need to switch the cop off. Maybe move it to the finder?
Risks
Involved components
participants_service.rb