maven: Update lang level detection

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

https://github.com/apache/netbeans-jackpot30/blob/86629e0ae5dfec0a64150b45cf124414e66dd740/cmdline/maven/src/main/java/org/netbeans/modules/jackpot30/maven/RunJackpot30.java#L48

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`.

Start at cmdline/maven/src/main/java/org/netbeans/modules/jackpot30/maven/RunJackpot30.java around line 48 and trace how the Maven plugin detects the language level. Compare handling of source, configured properties, release, and the runtime JDK; done means the plugin no longer relies only on source or an unexplained hardcoded fallback.

Written by the indexing model from the issue text.