Bump org.apache.maven.plugins:maven-site-plugin from 3.12.1 to 3.21.0

Bumps [org.apache.maven.plugins:maven-site-plugin](https://github.com/apache/maven-site-plugin) from 3.12.1 to 3.21.0.
- [Release notes](https://github.com/apache/maven-site-plugin/releases)
- [Commits](apache/maven-site-plugin@maven-site-plugin-3.12.1...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

@@ -201,7 +201,7 @@
     builds but scrambles the HTML output for the Changes and Jira report pages.
     See also https://lists.apache.org/thread/pgj1gd75hx1dhq9spnl3c70xxt07corx
     -->
-    <commons.site-plugin.version>3.12.1</commons.site-plugin.version>
+    <commons.site-plugin.version>3.21.0</commons.site-plugin.version>
     <commons.spotbugs.plugin.version>4.8.6.4</commons.spotbugs.plugin.version>
     <commons.spotbugs.impl.version>4.8.6</commons.spotbugs.impl.version>
     <commons.surefire-report.version>3.5.1</commons.surefire-report.version>