Bump org.apache.maven.plugins:maven-site-plugin from 3.20.0 to 3.21.0 (#220)

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
...

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Author: dependabot[bot]

pom.xml

@@ -55,7 +55,7 @@
         <maven.compiler.plugin.version>3.13.0</maven.compiler.plugin.version>
         <maven.jar.plugin.version>3.4.2</maven.jar.plugin.version>
         <maven.javadoc.plugin.version>3.11.1</maven.javadoc.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>3.5.2</maven.surefire.plugin.version>
     </properties>