KIP-620 — Deprecate ConsumerConfig#addDeserializerToConfig and ProducerConfig#addSerializerToConfig

Accepted Kafka 2.7 Client

Deprecates and removes `ConsumerConfig.addDeserializerToConfig()` and `ProducerConfig.addSerializerToConfig()` static methods that became dead code after the internal serializer/deserializer instantiation was refactored. These utility methods no longer served any purpose after the config-building path they supported was removed, but remained in the public API adding confusion.

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