Downgrade site-plugin to work with Fluido skin

(DOXIASITETOOLS-356)

Author: keeganwitt

pom.xml

@@ -395,7 +395,7 @@
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-site-plugin</artifactId>
-        <version>4.0.0-M16</version>
+        <version>3.21.0</version>
         <executions>
           <execution>
             <id>stage-for-scm-publish</id>