[MNG-6506] [MNG-6543] Upgrade Plexus Classworlds to 2.6.0
[MNG-6506] [MNG-6543] Upgrade Plexus Classworlds to 2.6.0

Enable plugins that use the new JPMS Java 9 API to work in Java 9+
apache/maven,mcculls/maven,mcculls/maven,olamy/maven,josephw/maven,apache/maven,olamy/maven,cstamas/maven,apache/maven,cstamas/maven,cstamas/maven,olamy/maven,josephw/maven,josephw/maven,mcculls/maven