Confirm that pending PDM count is displayed correctly in Release management dashboard

The Pending post-deployment migrations panel in the Release management dashboard should use the new metric that is posted from Kubernetes.

The name of the metric is identical before and after: delivery_metrics_pending_migrations_total. This issue is mainly to track that the panel works properly even after we stop posting the metric from the deploy VM and ensure that the panel does not end up showing no data at some point.

/label ~"group::delivery-deploy"  
/assign me
/blocked_by https://gitlab.com/gitlab-com/gl-infra/delivery/-/issues/21041
/blocked_by https://gitlab.com/gitlab-com/gl-infra/delivery/-/issues/21042