Add backend filtering for issue autocomplete

What does this MR do and why?

Adds a feature flag to filter issue autocomplete results on the backend so we do not have to return all open issues to the frontend

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. Open an issue / MR
2. Try autocompleting an issue by typing # in a Markdown field
3. Verify that only 5 results are returned from the backend and that requests are made every time you update the search term

Related to #289238 (closed)