Deprecation path for monitoring.cluster_alerts.allowedSpaces setting

[klacabane]

Summary

Starting 7.13 we added support for a monitoring.cluster_alerts.allowedSpaces setting which prevents automatic creation of stack monitoring alerts in spaces that are not allowed (see #99128).

As of now, the documentation of that setting is not accurate and should be updated for the subset of kibana versions that support it.
Since then we also reworked the way these alerts are created, and we now present a modal where the user can opt-in the creation of the alerts which makes that setting unnecessary.

AC

• update documentation for impacted 7.x versions update monitoring.allowedSpaces documentation #120362
• update cloud documentation https://github.com/elastic/cloud/pull/93686
• add a deprecation warning for 7.16+ [Stack monitoring] deprecation warning for allowedSpaces setting #120561
• remove the setting in 8.0 [Stack monitoring] remove support for monitoring.cluster_alerts.allowedSpaces #123229

[elasticmachine]

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

[elasticmachine]

Pinging @elastic/kibana-docs (Team:Docs)