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

Author: renovate[bot]

pom.xml

@@ -87,7 +87,7 @@
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-site-plugin</artifactId>
-                <version>3.20.0</version>
+                <version>3.21.0</version>
             </plugin>
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>