Refer users to Kafka community/support for compatibility information

[ppf2]

We attempted to be helpful here by linking to Kafka compatibility information for client and brokers (https://www.elastic.co/guide/en/logstash/current/plugins-inputs-kafka.html). Unfortunately, this page (https://cwiki.apache.org/confluence/display/KAFKA/Compatibility+Matrix) doesn't get updated frequently. For example, there is no information for 1.1 released months ago.

Given that Kafka client <-> broker compatibility is really part of the Kafka product, we should consider removing references to the Wiki page and simply instruct users to contact Kafka community/support to confirm compatibility information between Kafka client and broker versions. From LS's point of view, we simply have to include the Kafka client version used by each LS release (which we are already doing).

Same thing for the Kafka output plugin documentation.