[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:

* [x] Stack docs: https://www.elastic.co/guide/en/elastic-stack-overview/current/xpack-monitoring.html
* [x] Elasticsearch: https://www.elastic.co/guide/en/elasticsearch/reference/current/monitor-elasticsearch-cluster.html
* [x] 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