Decrease Cyclomatic Complexity threshold to 13

What does this MR do?

• refactor lib/gitlab/conflict/parser.rb:14:7
• refactor lib/gitlab/mail_room.rb:22:7
• decrease Cyclomatic Complexity threshold to 13

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

It's fourth step for #31362 (moved)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

#31362 (moved)

/cc @rymai @markglenfletcher @grzesiek @blackst0ne @rspeicher

Possible labels: Community Contribution backstage Edge static analysis