Address ambiguity now that Testcontainers has two classes named KafkaContainer

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`.