Add JDK 16+ workaround for Nexus Staging Maven Plugin
Plugin version 1.6.8 does not work on JDK 16+ without special MAVEN_OPTS
opening certain modules, because the XStream version used needs it. One
workaround is to use a more recent XStream version.
TODO: remove plugin dependency after OSSRH-66257, NEXUS-26993 are fixed.
Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>