[Grafana] rename thanos grafana datasource and remove old prometheus datasources
What
- Renames the old Global datasource to "Thanos - Historical Data"
- Removes Prometheus VM datasources which are being decommissioned.
Why
This is part of the decommissioning process for Thanos.
We recently migrated to Mimir and most of dashbaords are not using the new datasources.
Related Issue: gitlab-com/gl-infra/scalability#3566 (closed)