Test with Kafka 4 server

https://kafka.apache.org/blog

We have #296 to upgrade the client jar. So far, it seems like the v4 client works with the v3 server.

We use Docker images from Confluent to run the Kafka server. They have not yet published their v8 to support Kafka 4. We could consider finding other Docker images.

We may want to also continue testing with Kafka v3 server as well as v4.