Use iids as filter parameter

What does this MR do?

iids is used as parameter for filtering milestones by iid instead of iid so that we're consistent across the API.

Why was this MR needed?

API consistency

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 #27532 (closed)