kotsadm: "Configure Graphs" button on dashboard is misleading

The default value in the Configure Graphs button is https://prometheus.default.svc.cluster.local:9090, and while this is certainly a valid value if the end user is running prometheus in that namespace, I think this modal could use a little more explanation:

1. In the modal, we should highlight the current value that was auto-configured, if any (e.g. in a kURL cluster)
2. we should probably include some 1-2 sentence explanation of what this is for, e.g. If you have an in-cluster or out-of-cluster prometheus instance, you can add a URL to that instance here to display some graphs on the kotsadm admin console dashboard
3. we should consider hiding the configure graphs button entirely in kURL clusters, or changing the text to be more clear about "hey you probably shouldn't do this because we've already wired it up for you"

Start at the kotsadm dashboard's Configure Graphs modal and trace how the default Prometheus URL is populated, including in kURL clusters. Clarify the intended behavior for the auto-configured value, explanatory text, and button visibility or wording, then verify the dashboard presents the correct guidance and graph configuration options.