Streamline update policy of pluginRepository/repository of Maven Central in Super POM

The update policy is set to never for the Maven Central pluginRepository (https://github.com/apache/maven/blob/b23afbc9849d57b0a4ff0f2bdd3fff5520a95f7c/maven-model-builder/src/main/resources/org/apache/maven/model/pom-4.0.0.xml#L48) while it is the default (= daily) for the regular repository.

Although it would be nice to set both to never this does not work with Resolver 1.x as that treats metadata and artifacts the same (but in fact metadata is constantly changing even for release repositories like Maven Central). This situation should improve with Resolver 2.x (Maven 4) (https://issues.apache.org/jira/browse/MRESOLVER-377).