Introduce alert deprecation notice on metrics dashboard

As part of #327796 (closed), the alert functionality on the Metrics Dashboard is being deprecated. This change is happening in 14.0. The deprecation notice is already displaying on the docs page.

We need to display additional alerts in our UI letting users of the current alert functionality know the impact of this change in 13.12. We'll display this alert in two places:

1. On the metrics dashboard
2. On the alert list page

For this alert, we should utilize a warning alert.

WIP text (still needing approval from TW):

GitLab-managed Prometheus is deprecated and scheduled for removal. Following this removal, your existing alerts will continue to function as part of the new cluster integration. However, you will no longer be able to add new alerts or edit existing alerts from the metrics dashboard.