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] <[email protected]>

pom.xml

@@ -70,7 +70,7 @@
         <maven-gpg-plugin.version>3.2.7</maven-gpg-plugin.version>
         <maven-javadoc-plugin.version>3.10.1</maven-javadoc-plugin.version>
         <maven-project-info-reports-plugin.version>3.8.0</maven-project-info-reports-plugin.version>
-        <maven-site-plugin.version>3.20.0</maven-site-plugin.version>
+        <maven-site-plugin.version>3.21.0</maven-site-plugin.version>
         <maven-source-plugin.version>3.3.1</maven-source-plugin.version>
         <maven-surefire-plugin.version>3.5.1</maven-surefire-plugin.version>
         <versions-maven-plugin.version>2.17.1</versions-maven-plugin.version>