Update parent version to 21 for release to central portal

Author: ruhan1

pom.xml

@@ -21,7 +21,7 @@
   <parent>
     <groupId>org.commonjava</groupId>
     <artifactId>commonjava</artifactId>
-    <version>18</version>
+    <version>21</version>
   </parent>
 
   <groupId>org.commonjava.indy</groupId>