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

Bumps [org.apache.maven.plugins:maven-site-plugin](https://github.com/apache/maven-site-plugin) from 3.20.0 to 3.21.0.
- [Release notes](https://github.com/apache/maven-site-plugin/releases)
- [Commits](apache/maven-site-plugin@maven-site-plugin-3.20.0...maven-site-plugin-3.21.0)

---
updated-dependencies:
- dependency-name: org.apache.maven.plugins:maven-site-plugin
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

pom.xml

@@ -49,7 +49,7 @@
         <version.org.apache.maven-invoker-plugin>3.8.1</version.org.apache.maven-invoker-plugin>
         <version.org.apache.maven-resources-plugin>3.3.1</version.org.apache.maven-resources-plugin>
         <version.org.apache.maven-shade-plugin>3.6.0</version.org.apache.maven-shade-plugin>
-        <version.org.apache.maven-site-plugin>3.20.0</version.org.apache.maven-site-plugin>
+        <version.org.apache.maven-site-plugin>3.21.0</version.org.apache.maven-site-plugin>
         <version.org.apache.maven.plugins.maven-checkstyle-plugin>3.6.0</version.org.apache.maven.plugins.maven-checkstyle-plugin>
         <version.org.codehaus.animal-sniffer-maven-plugin>1.24</version.org.codehaus.animal-sniffer-maven-plugin>
         <version.org.eclipse.lifecycle-mapping>1.0.0</version.org.eclipse.lifecycle-mapping>