Use new auth filter in merge requests

What does this MR do and why?

This MR introduces using the new query builder authorization filter to merge request search behind a FF.

Changes (generated by Duo)

• Created feature flag search_advanced_merge_requests_new_auth_filter to enable new authorization filter for merge request searches
• Updated MergeRequestQueryBuilder to conditionally use by_search_level_and_membership (new filter) when feature flag is enabled, or by_project_authorization (old filter) when disabled
• Added comprehensive specs to test both feature flag enabled and disabled scenarios, ensuring proper filter selection and backward compatibility
• Implemented gradual rollout mechanism for improved query performance while maintaining safe rollback capability

References

Relates to issue #491211

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. enable elasticsearch in gdk
2. perform searches at global, group, and project level for * (note how many merge requests you get)
3. enable the FF search_advanced_merge_requests_new_auth_filter
4. perform the same searches, make sure the numbers match
5. repeat with a non-admin user
6. repeat with an anonymous user

MR acceptance checklist

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