Deprecate monitoring.ui.elasticsearch settings in favor of CCS

[matschaffer]

https://www.elastic.co/guide/en/kibana/current/monitoring-settings-kb.html#monitoring-general-settings includes information about setting monitoring.ui.elasticsearch.hosts to "enables you to use a single Kibana instance to search and visualize data in your production cluster as well as monitor data sent to a dedicated monitoring cluster."

This is likely a bad idea since it only works if you're also syncing user credentials between the production and monitoring clusters.

I suspect the configuration pre-dates CCS which should now provide a much safer way to access both production and monitoring data from the same kibana.

I'd recommend we deprecate the setting in favor CCS - perhaps pointing to https://www.elastic.co/guide/en/elasticsearch/reference/current/remote-clusters-privileges.html#clusters-privileges-ccs-kibana

[elasticmachine]

Pinging @elastic/infra-monitoring-ui (Team:Infra Monitoring UI)

[matschaffer]

Ah, the tip on https://www.elastic.co/guide/en/kibana/master/monitoring-data.html calls it out. Wondering if we should update that as well.