JEP 181 brings a new bytecode format for nest mates. ASM API 7 supports
this, but API 7 is still experimental. Attempting to load a Java 11
class that contains nest mates will cause an
UnsupportedOperationException, as API 6 can't handle them. This doesn't
really require any changes to the rest of the code, since this is just
related to ASM's visitor.
We want to use the stable API 6 for all other plugins, only using the
experimental API when required, so we check the class version first.
This should be removed as soon as ASM API 7 is stable.
