Update Kafka Output documentation to indicate a protocol version of 2.1.0 is required for Kafka 4.0

[strawgate]

Describe the enhancement:
Upcoming Kafka 4.0 has a minimum protocol version of 2.1.0. We should indicate this in our documents.

Describe a specific use case for the enhancement or feature:

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[khushijain21]

completed by #41540