Update dependency org.apache.maven.plugins:maven-site-plugin to v3.21.0

Author: renovate[bot]

pom.xml

@@ -73,7 +73,7 @@
         <!-- site lifecycle, see https://maven.apache.org/ref/current/maven-core/lifecycles.html#site_Lifecycle -->
         <plugin>
           <artifactId>maven-site-plugin</artifactId>
-          <version>3.9.1</version>
+          <version>3.21.0</version>
         </plugin>
         <plugin>
           <artifactId>maven-project-info-reports-plugin</artifactId>