Warn strongly against using Prometheus Exporter for Couchbase Server 7.x+

Using the Prometheus Exporter on a 7.x cluster is known to cause elevated CPU usage, sometimes to the point of causing serious issues (see linked CBSEs).

The current docs page does recommend using the native endpoint over the Prometheus Exporter, but the tone of the page is generally that this is still user preference:

You can set up the Autonomous Operator to use the Couchbase Exporter or Couchbase Server's native support for metrics collection ... Couchbase native support is available for Couchbase Server versions 7.0 or higher and is the recommended way for collecting metrics with Prometheus.

We should make it clear that the Couchbase Prometheus Exporter is a legacy method for accessing Couchbase metrics and explicitly warn against using this for versions of Couchbase Server from version 7 onwards (we can state that its usage with these CB Server versions is unsupported). The native support for metrics is the only supported method from version 7 onwards.

There is ongoing debate over whether/when the Exporter may be deprecated altogether (see K8S-3112), but its only use case now should be for any remaining pre-7 clusters.