Allow Kibana to run against an older Elasticsearch version at the patch level

[epixa]

Following discussion about #8267, users should be able to run a newer patch version of Kibana against an older patch version of Elasticsearch, and we should log an appropriate warning on startup whenever they do. This is the exact same behavior that we already provide when doing the opposite: running a newer patch version of Elasticsearch against an older patch version of Kibana.

This will allow the application of critical bug fixes and such to Kibana without having to do a patch upgrade across many ES nodes. If a bug fix in Kibana requires behavior from a specific ES version, we should be enforcing that specific version anyway, but that should be the exception to the rule.

This change would mean that the above compatibility chart in our readme would change so that "ES patch number is older." would log a warning instead of causing a fatal error.

[megastef]

Where can I get Kibana 2.4 for Elasticsearch 2.4? ;)
Fun aside, where can I find the list with real version numbers? Would Kibana 5 work with 2.4 and 2.3?

[epixa]

@megastef This issue only applies for 5.0.0+. The version compatibility matrix prior to 5.0 is impossible to verify or map in this way.

Kibana 5.0 will not work against any prior major version of Elasticsearch. In other words, Kibana 5 is only compatible with Elasticsearch 5.