Upgrade to new major version of Kafka

[jskeet]

We're depending on v1.x of Confluent.Kafka, last released in December 2022.

The release notes for the first 2.x release state:

OpenSSL 3.0.x upgrade in librdkafka requires a major version bump, as some legacy ciphers need to be explicitly configured to continue working, but it is highly recommended NOT to use them. The rest of the API remains backward compatible.

Unfortunately we can't tell whether consumers will be using those legacy ciphers.

@captainsafia do you have experience with Kafka, or know what other projects have done in terms of this? We can take a new major for Kafka like we have for MQTT if we need to - and document it with the release note above.