Update migration to not delete existing Grafana alerts (#39541)
Update migration to not delete existing Grafana alerts (#39541)

* keep existing unified alert rules untouched
* move silences and other alertmanager files to the organization directory (only if it is a single organization deployment)
* assign the existing notification settings and routes to the first organization
* create default notification settings for each organization in the case of multi org deployment
grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana