HADOOP-18598. maven site generation doesn't include javadocs. (#5319)

Reviewed-by: Chris Nauroth <cnauroth@apache.org>

pom.xml

@@ -95,7 +95,7 @@ xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/x
 
     <!-- maven plugin versions -->
     <maven-deploy-plugin.version>2.8.1</maven-deploy-plugin.version>
-    <maven-site-plugin.version>3.11.0</maven-site-plugin.version>
+    <maven-site-plugin.version>3.9.1</maven-site-plugin.version>
     <maven-stylus-skin.version>1.5</maven-stylus-skin.version>
     <maven-antrun-plugin.version>1.7</maven-antrun-plugin.version>
     <maven-assembly-plugin.version>2.4</maven-assembly-plugin.version>