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

@@ -92,7 +92,7 @@
     <maven.pmd.version>3.17.0</maven.pmd.version>
     <maven.source.version>3.3.0</maven.source.version>
     <maven.install.version>3.1.3</maven.install.version>
-    <maven.site.version>3.20.0</maven.site.version>
+    <maven.site.version>3.21.0</maven.site.version>
     <maven.clean.version>3.4.0</maven.clean.version>
     <maven.jar.version>3.4.2</maven.jar.version>