switch to Java 11

[rPraml]

Some tools (like eclipse https://bugs.eclipse.org/bugs/show_bug.cgi?id=536273 ) cannot detect the module-info.class in META-INF/versions/9.
I think it would be a good idea to switch here completely to java 11.
(Note: multiple PRs will follow)

[rPraml]

Please wait. I think we run in a never ending story here, as other multi-version-jars (like fasterxml) will have similar problems

[rPraml]

@rbygrave FYI: This bug is reported here: https://bugs.eclipse.org/bugs/show_bug.cgi?id=577790

We currently use the eclipse compiler (ecj) in nearly all of our projects as it supports multi-threading and is almost twice as fast as javac. And it behaves exactly the same as in eclipse. (There are some code places, where the javac expects excplicit casts, while ecj does not)

We do not use JPMS, yet, but we intend to use it. We are also seriously considering moving from Eclipse/Ecj to IntelliJ/Javac due to the issues I've seen over the past few weeks...

You may accept or close this PR - This change is no longer necessary from our side.

[rbygrave]

Ok, closing.