* [kotlin-server][javalin6] Add Javalin 6 support
Javalin 5 support was added in 13edc5d. Javalin 6 has been released, with some breaking changes. Let's add a new supportedLibrary to not break existing users of Javalin 5.
https://javalin.io/migration-guide-javalin-5-to-6
* Fix Gradle config and don't include JVM 8 CI anymore (JVM 11 is the minimum for Javalin)
* Update docs
* Fix optional query parameter handling and turn into expected type
