prevent filtering Issues by multiple milestones, authors, or assignees

What does this MR do?

Filter issues dropdown to exclude existing filters that are 1:1

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

• Changed getSearchInput to getSearchQuery

Why was this MR needed?

Can't have multiple anything except labels

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

Added EE compat MR https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1279/

What are the relevant issue numbers?

Closes #27174 (closed)