Kotlin should be part of the java transitive dependency tree

[jottinger]

For the Java dependency, kotlin should be a transitive dependency; it doesn't have to be a recent Kotlin but it should be there. An alternative is to specify in the docs that you need to manually add Kotlin as a dependency - but having a default transitive dependency is better.

If users require a newer Kotlin (suppose they use 1.5 and you used 1.4?) that's okay, because the build tool will use the more recent version.