Make BCEL classpath utility recognise Java 15, fixing many tests

Make BCEL classpath utility recognise Java 15, fixing many tests

Inside org.aspectj.apache.bcel.util.ClassPath.getClassPath(), some JVM
version matching occurs which previously did not include Java 15.
Technically, AspectJ 1.9.6 does not support Java 15, but on GitHub
Actions there is a build job running on a JVM 15. This change should at
least make the weaver tests pass, making that test job more meaningful.

This fixes test errors like

java.lang.ClassCastException:
  class org.aspectj.weaver.MissingResolvedTypeWithKnownSignature
  cannot be cast to class
  org.aspectj.weaver.ReferenceType
  (org.aspectj.weaver.MissingResolvedTypeWithKnownSignature and
  org.aspectj.weaver.ReferenceType are in unnamed module
  of loader 'app')

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>