[Stack Monitoring] Cleanup references to deprecated config

[neptunian]

• Remove reference to monitoring.cluster_alerts.email_notifications.email_address in settings collector (internal collection)
• Remove settings collector that references the above. It used to be read by the cluster alert watches in ES but those are gone now. (Note: This may also still exist in MB and can be removed)
• Remove cluster_alerts.email_notifications.enabled and monitoring.cluster_alerts.email_notifications.email_address from our config because they have been unused since 7.12 after removing watcher alerts. ref: https://github.com/elastic/kibana/pull/87377 https://github.com/elastic/kibana/pull/91272 . We ought to have removed these at 8.0 but we didn't. Can we safely do that now whilst removing the deprecation warnings?
• Remove deprecation warnings.
• Remove this incorrect warning
• Update docs!

[elasticmachine]

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