[Stack Monitoring] Deprecate internal collection topics

We now have the ability to monitor Elasticsearch, Kibana, Logstash, Beats, and APM Server with Metricbeat. The previous method of monitoring stack products, using collection code internal to each product that shipped monitoring data to a custom Monitoring Bulk API endpoint, is now deprecated.

We should reflect this deprecation in documentation as well, specifically in these areas:

• Stack docs: https://www.elastic.co/guide/en/elastic-stack-overview/current/xpack-monitoring.html
• Elasticsearch: https://www.elastic.co/guide/en/elasticsearch/reference/current/monitor-elasticsearch-cluster.html
• Kibana: https://www.elastic.co/guide/en/kibana/current/configuring-monitoring.html
• Beats: https://www.elastic.co/guide/en/beats/heartbeat/current/monitoring.html (Using Heartbeat as a sample here, but applicable to all Beats and APM Server documentation)
• Logstash: TBD; PR introducing Metricbeat collection is currently WIP: https://github.com/elastic/logstash/pull/11033

Start with the unchecked Beats monitoring page at https://www.elastic.co/guide/en/beats/heartbeat/current/monitoring.html and compare it with the checked Stack, Elasticsearch, and Kibana documentation links. Update the applicable Beats and APM Server documentation to reflect the deprecation of internal collection topics; determine the Logstash documentation location once the referenced PR is no longer WIP.