KIP-174 — Deprecate and remove internal converter configs in WorkerConfig

Accepted Kafka 2.0 Connect

Deprecates and then removes `internal.key.converter` and `internal.value.converter` from Kafka Connect `WorkerConfig`, hardcoding `JsonConverter` as the only supported internal format. These configs were exposed prematurely before classloader isolation existed; they have caused more operator confusion than flexibility, and no one needs to swap internal converters in practice.

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