Bump org.apache.maven.plugins:maven-site-plugin from 3.20.0 to 3.21.0

Author: dependabot[bot]

pom.xml

@@ -136,7 +136,7 @@
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-site-plugin</artifactId>
-        <version>3.20.0</version>
+        <version>3.21.0</version>
       </plugin>
     </plugins>
   </build>