Unnecessary and unactionable warning given when connecting to an Elasticsearch version > 6

[robbavey]

When connecting to an Elasticsearch cluster 6.x and above, the following warning message is emitted at Logstash startup:

Detected a 6.x and above cluster: the `type` event field won't be used to determine the document _type {:es_version=>7}

As, we are preparing to release 9.0 of the Elastic Stack, this warning message is likely unnecessary at this point.