Merge pull request #3470, Apache parent pom version is updated to 21.

Author: kamaci

pom.xml

@@ -22,7 +22,7 @@
     <parent>
         <groupId>org.apache</groupId>
         <artifactId>apache</artifactId>
-        <version>19</version>
+        <version>21</version>
     </parent>
 
     <groupId>org.apache.dubbo</groupId>