Update to Java 12+

[bugabinga]

Update to Java 12+ while still using the class path (no module path).
Configure the necessary build options in maven to be able to compile legacy code if necessary.
Resolve issues resulting in breaking API changes from newer Java versions.

Try to keep refactoring and new idioms (e.g. locally inferred variable types) to a minimum, however note all refactoring opportunities in a new issue.