Multi-cluster support in Grafana Dashboards should be generic

[JFlath]

At present, we expose the option to filter between multiple Redpanda Clusters in some dashboards based on a label redpanda_cloud_data_cluster_name, which has been carried forward from previous/other uses of the dashboards.

We should make this more generic and potentially document its use to the point of examples of how to add appropriate labels to clusters' metrics when scraping.