Allow to disable/hide Grafana Managed Alerts

### Discussed in https://github.com/grafana/grafana/discussions/65239 <div type='discussions-op-text'> ^{Originally posted by **kubicgruenfeld** March 22, 2023} Our users get confused by "Grafana Managed Alerts" since they should add alerts only in Loki and Mimir since our Grafana is not high available, but Loki and Mimir is. There is even a hint in the UI: Select “Grafana managed” unless you have a Mimir, Loki or Cortex data source with the Ruler API enabled.", but you can not disable it for now. We would be happy to provide a PR to allow disabling "Grafana Managed Alerts".</div>