update to Kafka 2.6.0

[ManuelWeiss]

I've updated most dependencies and fixed all deprecations as well. However, I'm running into this error:

[error] Test suite cakesolutions.kafka.ConsumerProducerIntSpec failed with java.lang.NoClassDefFoundError: scala/math/Ordering$$anon$7.
[error] This may be due to the ClassLoaderLayeringStrategy (Flat) used by your task.
[error] To improve performance and reduce memory, sbt attempts to cache the class loaders used to load the project dependencies.
[error] The project class files are loaded in a separate class loader that is created for each test run.
[error] The test class loader accesses the project dependency classes using the cached project dependency classloader.
[error] With this approach, class loading may fail under the following conditions:
[error]
[error]  * Dependencies use reflection to access classes in your project's classpath.
[error]    Java serialization/deserialization may cause this.
[error]  * An open package is accessed across layers. If the project's classes access or extend
[error]    jvm package private classes defined in a project dependency, it may cause an IllegalAccessError
[error]    because the jvm enforces package private at the classloader level.
[error]
[error] These issues, along with others that were not enumerated above, may be resolved by changing the class loader layering strategy.
[error] The Flat and ScalaLibrary strategies bundle the full project classpath in the same class loader.
[error] To use one of these strategies, set the  ClassLoaderLayeringStrategy key
[error] in your configuration, for example:
[error]
[error] set scalaKafkaClient / Test / classLoaderLayeringStrategy := ClassLoaderLayeringStrategy.ScalaLibrary
[error] set scalaKafkaClient / Test / classLoaderLayeringStrategy := ClassLoaderLayeringStrategy.Flat

I've tried the fix suggested there but it didn't make any difference.