Decrease Metrics/CyclomaticComplexity threshold to 15

What does this MR do?

• Decrease Cyclomatic Complexity threshold to 15

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

check Cyclomatic Complexity in CE and EE (for now there are no offences in CE or EE for this threshold)

Why was this MR needed?

It's second 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

Possible labels: ~"Community Contribution" ~backstage ~Edge