Update parent to pom-scijava-base 21.0.0

A vital change here is the update of Maven Central publishing
mechanism from the obsolete nexus-staging-maven-plugin (OSSRH)
to central-publishing-maven-plugin (Central Portal).

Author: ctrueden

pom.xml

@@ -6,7 +6,7 @@
 	<parent>
 		<groupId>org.scijava</groupId>
 		<artifactId>pom-scijava-base</artifactId>
-		<version>20.1.0</version>
+		<version>21.0.0</version>
 		<relativePath />
 	</parent>