Teach maven-helper about Maven Central

The nar-maven-plugin is now deployed exclusively to Maven Central
(actually, OSS Sonatype, but that is mirrored to Maven Central).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
bump-pom-scijava: the groupId of nar-maven-plugin has changed

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>