fail fast and obviously if building on something other than Java 8

The 5.16.x build requires you build using Java 8. It fails if run on e.g 11, which the main/5.17.x build requires is used, and people increasingly have as their default JDK regardless.

Add an enforcer check (or rather, replace the ineffective existing one) to make the build fail immediately and in a quickly obvious "you must build using Java 8" way.