Speed up group milestones show page

What does this MR do?

It adds an index and reduces number of queries done for this page

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

N/A

Why was this MR needed?

Speeding up things. It should bring down the load time from ~22 to ~6 seconds.

Screenshots (if relevant)

N/A

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 you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

#22454 (closed)