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

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.
