DeadLetterQueueReporter leaks KafkaProducer instance

The DeadLetterQueueReporter (introduced by KAFKA-6738) creates a KafkaProducer to report errors to Kafka, but does not clean up the producer, leaving many idle network threads open after tasks are stopped.

Reproduction steps:
Start a task that has a non-empty DLQ topic name
Stop the task
Observe the list of running threads

Expected result:
There is no thread related to the stopped task's DLQ left running

Actual result:
There is a thread named something like kafka-producer-network-thread | connector-dlq-producer-task-0" #1234 left running