Rolling upgrade from 6.5.4 to 6.6.1 do not work well

[vbohata]

Kibana version:
6.6.1

Elasticsearch version:
6.6.1

Server OS version:
Centos 7

Describe the bug:
Rolling upgrade made all the Kibana unavailable until the cluster including all Kibanas has been fully upgraded. I did rolling upgrade of ES. Worked well after that. Then I upgraded Kibana, restarted it and the remaining Kibana including the upgraded one started to report errors like "Multiple versions of Kibana are running against the same Elasticsearch cluster, unable to authorize user." and returning 500. After upgrading all the Kibanas I had to restart them to make them work again.
This behaviour is new, I did not have issues during previous upgrades. This bug makes life upgrade impossible.

[elasticmachine]

Pinging @elastic/kibana-operations

[tylersmalley]

Kibana does not currently support rolling upgrades and shutting down the existing instances is mentioned in the upgrade procedure.

This is something we are actively working towards. #41795 #66056