maven: Update lang level detection

[mbien]

the maven plugin is currently only checking the source element, not the property or release version.

netbeans-jackpot30/cmdline/maven/src/main/java/org/netbeans/modules/jackpot30/maven/RunJackpot30.java

Line 48 in 86629e0

Xpp3Dom source = sourceLevelConfiguration.getChild("source");

If the property is left undefined, it would be better to use the runtime JDK version (max supported?) instead of a hardcoded fallback, analog to how javac works or the maven-compiler-plugin.