Make Drools to run on JDK11

Justification: https://blog.codefx.org/java/java-11-migration-guide/
Other good resource: https://winterbe.com/posts/2018/08/29/migrate-maven-projects-to-java-11-jigsaw/

Note: our primary build is still JDK 8, we just want to be JDK 11 friendly.