Use new auth filter in milestones

What does this MR do and why?

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

Changes

• Created feature flag search_advanced_milestones_new_auth_filter to enable new authorization filter for milestone searches
• Updated MilestoneQueryBuilder 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

Related #491213

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 milestones you get)
3. Enable the FF search_advanced_milestones_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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