Update to Java 11

Due to the recent changes in the Jenkinsfiles which are now streamlined, this plugin is not tested anymore with toolchains. That means, we currently do not test with Java 8 running Maven and Java 11 Toolchains.

However, as this is an improbable and unlikely scenario, we can do the following:

• Require Java 11 for the plugin
• Move classes from the java9 source folder to the java source folder

We could move to Java 9 only, but I do not see any benefit from using a long-dead non-LTS version as a baseline.