Backend: Allow archived projects to be excluded from milestones search results

Behind a feature flag: search_milestones_hide_archived_projects with a user actor

If the feature flag is enabled: exclude archived projects by default. If the param include_archived is true, include archived projects.

Should be done for basic and advanced global and group search. Project search results should not be changed. API will be done separately in #413239.

Blueprint MRs:

• Exclude archived projects in search results by ... (!121981 - merged)
• Exclude archive projects by default behind a fe... (!122332 - merged)