build(deps): update dependency org.apache.maven.plugins:maven-site-plugin to v3.21.0 (#2004)

Author: renovate-bot

google-http-client-bom/pom.xml

@@ -141,7 +141,7 @@
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-site-plugin</artifactId>
-        <version>3.12.1</version>
+        <version>3.21.0</version>
         <configuration>
           <skip>true</skip>
         </configuration>

pom.xml

@@ -388,7 +388,7 @@
         <plugin>
           <groupId>org.apache.maven.plugins</groupId>
           <artifactId>maven-site-plugin</artifactId>
-          <version>3.12.1</version>
+          <version>3.21.0</version>
         </plugin>
         <plugin>
           <groupId>org.apache.maven.plugins</groupId>