According to an old question about How to fix java.lang.UnsupportedClassVersionError...
The fix is to update my JRE, and my PATH variable, so they point to the latest JRE and JDK... I've done that...
I'm at JRE 8 version 211, apparently the latest, with the latest JDK, JDK 12...
So why is "java" saying that my Java Runtime can only run on class files up on "version 52", while my class files are compiled with that JDK, are "class file version 56"?
Since everything's the latest version, shouldn't that mean that I can run what I compile?