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

@@ -96,7 +96,7 @@
     <maven.project.info.reports.plugin.version>3.7.0</maven.project.info.reports.plugin.version>
     <maven.release.plugin.version>3.1.1</maven.release.plugin.version>
     <maven.resources.plugin.version>3.3.1</maven.resources.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.spotbugs.plugin.version>4.8.6.5</maven.spotbugs.plugin.version>
     <maven.surefire.plugin.version>3.5.1</maven.surefire.plugin.version>