Upgrade project to Java 11

[Petter Reinholdtsen]

[Thomas Sødring]
> Anybody else have any thought?

I think we will just have to focus exclusively on OpenJDK in the future.
Is it possible to get the building and working with version 8, 9, 10 and
11, or will "migrating" to version 11 break our code for older versions?

Java 11 is not available in any stable Linux distribution, and we should
try our best to make sure Nikita will work with the stable releases of
at least Debian and Redhat, I believe.

--
Happy hacking
Petter Reinholdtsen