Bump org.apache.maven.plugins:maven-site-plugin from 3.12.1 to 3.21.0 (#2251)

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](https://github.com/apache/maven-site-plugin/compare/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
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

pom.xml

@@ -75,7 +75,7 @@
         <maven.enforcer.plugin.version>3.5.0</maven.enforcer.plugin.version>
         <flatten.maven.plugin.version>1.6.0</flatten.maven.plugin.version>
         <maven.scm.plugin.version>2.1.0</maven.scm.plugin.version>
-        <maven.site.plugin.version>3.12.1</maven.site.plugin.version>
+        <maven.site.plugin.version>3.21.0</maven.site.plugin.version>
 
         <license.tool.plugin.version>1.1.0</license.tool.plugin.version>
         <license.maven.plugin.version>4.5</license.maven.plugin.version>