* Remove alerting feature flag from advanced settings
Make it so the alerting UI in Observability can only be turned on and off in kibana.yml with `xpack.observability.enableAlertingExperience`.
We no longer:
* Allow turning this on and off for the current space in the advanced settings in Kibana
* Allow turning this on on cloud
* Send telemetry on whether or not the flag is enabled
Add some docs in READMEs as well.