MINOR: KafkaProducerTest - Fix resource leakage and replace explicit invocation of close() method with try with resources

[pramithas]

In few tests, producer.close() method was not called. Thus, wrapped the logic of producer creation inside try with resources.

Also, wherever the resources were closed with explicit invocation of close() method, I replaced those with try with resources.

All the tests are passing after the changes.

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)