added some javadoc

Signed-off-by: Laszlo Hornyak <laszlo.hornyak@gmail.com>
refactored kotlin version to property

Signed-off-by: Laszlo Hornyak <laszlo.hornyak@gmail.com>
upgraded to kotlin 0.8.11

Signed-off-by: Laszlo Hornyak <laszlo.hornyak@gmail.com>