[Docs] Kibana deprecation

Elasticsearch operator was deprecated with Logging 5.4.3, but there is no mention about the deprecation of Kibana.
As Kibana is needed in the EFK stack but not needed with Loki, the documentation should mention that Kibana is also deprecated and will be removed in a future release.
 

 [1] https://docs.openshift.com/container-platform/4.12/logging/cluster-logging-release-notes.html#openshift-logging-elasticsearch-dep