Move plexus-xml to the provided scope

This causes an error during the plugin descriptor generation/validation.
Both maven 3 and 4 provide export these classes, so the plexus-xml artifact itself should be flagged as provided.

pom.xml

@@ -81,6 +81,7 @@ under the License.
       <groupId>org.codehaus.plexus</groupId>
       <artifactId>plexus-xml</artifactId>
       <version>4.0.0</version>
+      <scope>provided</scope>
     </dependency>
 
     <!-- dependencies to annotations -->