Kafka-Streams-Scala for Scala 3

Kafka-Streams-Scala is currently not available for Scala 3. The required changes to compile the codebase with Scala 3 are trivial, so it's mostly about how the build system needs to change.

I've created a PR to enable separate configuration of the Scala version for the Kafka Core and Kafka-Streams-Scala.

https://github.com/apache/kafka/pull/15338

This is necessary because the Kafka-Streams-Scala Tests depend on Core, which hasn't been ported to Scala 3 yet. The most important thing to figure out is probably how CI and publishing should work – I'm not familiar with Kafka's CI setup, so I'm going to need some input from the Kafka team regarding this.