javadoc error when building with JDK11

When we build using JDK11 we get javadoc errors. For example building ArjunaCore/arjunacore:

javadoc: error - The code being documented uses modules but the packages defined in https://docs.oracle.com/javase/8/docs/api/ are in the unnamed module.

More context is here: https://bugs.java.com/bugdatabase/view_bug.do?bug_id=8212233
A fix is to configure the javadoc plugin (org.apache.maven.plugins:maven-javadoc-plugin) to use <source>8</source> in the plugin configuration section.