Use correct milestones for filtering on Group Issues

What does this MR do?

This allows users to use correct milestones for filtering on Group Issues on Web UI.

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

No.

Why was this MR needed?

To fix bug #34116 (closed).

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [n/a] Documentation created/updated
• [n/a] 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?

Closes #34116 (closed)