Upgrade to Plexus Utils 4.0.1 and downgrade to Plexus XML 3.0.0

See https://github.com/codehaus-plexus/plexus-utils/releases/tag/plexus-utils-4.0.1

Even if we did not notice any problem with plexus-xml 4, the documentation has been updated with:

keep plexus-xml to 3 if you want Maven 3 compatibility, as plexus-xml 4 works only in Maven 4.