KIP-1193 — Deprecate MX4j support

Accepted Kafka 4.2 Metrics

Deprecates MX4J support (the kafka_mx4jenable JVM flag) in Kafka 4.2 and removes it in 5.0. MX4J is a library for exposing JMX over HTTP that was last released in 2006, is completely undocumented in Kafka's official docs, and has had no reported users since a 2016 mailing list query, yet it adds maintenance burden and potential security exposure.

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