Add Metrics Dashboard starred dashboard API to FF

What does this MR do and why?

This MR gates the metrics dashboard starred dashboards API behind the remove_monitor_metrics feature flag. Frontend is done, but we also want to deprecate/remove any API access.

Related issue: Metrics: Delete API entrypoints & helpers (#397138 - closed) (one of many MRs)

Context for removal:

• The Metrics Dashboard feature is being removed in %16.0.
  • Feature docs: https://docs.gitlab.com/ee/operations/metrics
  • Removal issue: Monitor: Metrics Deprecation and Removal (&10107 - closed)
• Most of the metrics dashboard frontend is currently behind the remove_monitor_metrics flag, but the flag has not yet been enabled. It needs to be enabled by default in time for %16.0.
  • The entire frontend should be behind the flag in order to enable it by default, but the API can merge after - as long as we make all breaking changes within %16.0.
  • Rollout issue: https://gitlab.com/gitlab-org/gitlab/-/issues/399248+
  • Tracking issue: Metrics: hide deprecated modules behind a featu... (#399231 - closed)
  • Enable-by-default MR: Set remove_monitor_metrics flag to true (!119989 - merged)

All related backend MRs to flag all metrics dashboard endpoints

• Add Metrics Dashboard annnotations API to FF (!120286 - merged)
• Add Metrics Dashboard starred dashboard API to FF (!120288 - merged)
• Add Metrics Dashboard templates viewer to FF (!120289 - merged)
• Add Metrics Dashboard environments API to FF (!120299 - merged)
• Add Metrics Dashboard settings API to FF (!120305 - merged)
• Add Metrics Dashboard GraphQL alert queries to FF (!120307 - merged)
• Add Metrics Dashboard embed pipeline to FF (!120310 - merged)
• Add Metrics Dashboard for internal dashboard en... (!120313 - merged)
• Add Metrics Dashboard base API to FF (!120316 - merged)

How to set up and validate locally

Request	Response before	Response after
curl "http://gdk.test:3000/api/v4/projects/9/metrics/user_starred_dashboards?dashboard_path=config%2Fprometheus%2Fcommon_metrics.yml" \ --header "Authorization: Bearer $TOKEN" \ --request POST	{ "id":1, "dashboard_path":"config/prometheus/common_metrics.yml", "user_id":1, "project_id":9 }	{"message":"404 Not Found"
curl "http://gdk.test:3000/api/v4/projects/9/metrics/user_starred_dashboards?dashboard_path=config%2Fprometheus%2Fcommon_metrics.yml" \ --header "Authorization: Bearer $TOKEN" \ --request DELETE	{"deleted_rows":1}	{"message":"404 Not Found"}

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.