Update Grafana dashboards to use Metrics Collector for Apache Cassandra

[ossarga]

DataStax has recently open sourced a new metrics collector for Apache Cassandra.
It's a drop in solution and comes with Prometheus dashboards and works with all versions
between 2.2 to 4.0 alpha.

Blog: https://www.datastax.com/blog/2020/05/monitoring-apache-cassandratm-made-simple
GitHub: https://github.com/datastax/metric-collector-for-apache-cassandra

We will still need to keep the current dashboards for Cassandra versions lower than 2.2