configure javadoc-plugin depending on JDK version
configure javadoc-plugin depending on JDK version

The previously configured "--release" flag is not available on JDK8.

Also de-duplicate javadoc-plugin configuration in <reporting> block.

LearnLib/automatalib,misberner/automatalib,LearnLib/automatalib,misberner/automatalib,LearnLib/automatalib,LearnLib/automatalib