I am bumping the required JAVA SDK to 11 since it simplifies its usage
which results in improved maintanability. It also eliminates the need to
use bootclasspath.

- Merge VTK_JAVA_TARGET_VERSION and VTK_JAVA_SOURCE_VERSION to
  VTK_JAVA_RELEASE_VERSION.
- RENAME VTK_JAVA_INSTALL to VTK_JAVA_BUILD_MAVEN_PKG.
- Update Java build instructions

Co-authored-by: Ben Boeckel <ben.boeckel@kitware.com>