Default enable use_upsert_query_for_mr_metrics FF

What does this MR do and why?

This MR enables the use_upsert_query_for_mr_metrics feature flag by default. The flag is already enabled globally on PRD since 14.3.

Rollout issue: #339677 (closed)

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.

app/models/merge_request.rb

@@ -1869,7 +1869,7 @@ def banzai_render_context(field)
 
   override :ensure_metrics
   def ensure_metrics
-    if Feature.enabled?(:use_upsert_query_for_mr_metrics)
+    if Feature.enabled?(:use_upsert_query_for_mr_metrics, default_enabled: :yaml)
       MergeRequest::Metrics.record!(self)
     else
       # Backward compatibility: some merge request metrics records will not have target_project_id filled in.