Add app/models/concerns/issuable.rb to the list of files owned by group::code review

Summary

As part of the incident prompting this FCL, it was identified that the changes to file app/models/concerns/issuable.rb should have been flagged for approval by groupcode review . This file is not listed in CODEOWNERS but perhaps should be.

Implementation guide

• Update CODEOWNERS to mark app/models/concerns/issuable.rb as belonging to groupcode review
• Optional: consider whether another team is the correct owner based on the discussion here