Address ambiguity now that Testcontainers has two classes named KafkaContainer

[wilkinsona]

Testcontainers 1.19.8 introduces org.testcontainers.kafka.KafkaContainer, for using Apache's Kafka Docker image (the existing KafkaContainer uses Confluent's). When we upgrade to 1.19.8, we'll need to make some updates to the docs to be clear that we only support Confluent's at the moment. #40695 will add support for the new KafkaContainer.