WIP: Decrease abc size in rubocop.yml

What does this MR do?

Decrease threshold for ABC size metric to 59.89 ABC metric for MergeRequest::BuildService#execute now is about 38 so I think that #15562 (closed) can be closed.

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

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

fixes #15562 (closed)