Set Java level

Apparently the NBM mojo does not automatically define `OpenIDE-Module-Java-Dependencies` based on the Java level used by other mojos (https://github.com/apache/netbeans-mavenutils-nbm-maven-plugin/blob/17cb08756bd1f8f92ed8a02545c8dd317b26c603/src/main/java/org/apache/netbeans/nbm/NetBeansManifestUpdateMojo.java#L449-L452 or thereabouts), so this needs to be inserted manually. CC @JaroslavTulach