Bump org.apache.maven.plugins:maven-site-plugin from 3.7.1 to 3.21.0

Bumps [org.apache.maven.plugins:maven-site-plugin](https://github.com/apache/maven-site-plugin) from 3.7.1 to 3.21.0.
- [Release notes](https://github.com/apache/maven-site-plugin/releases)
- [Commits](apache/maven-site-plugin@maven-site-plugin-3.7.1...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>

Author: dependabot[bot]

pom.xml

@@ -111,7 +111,7 @@
           <artifactId>maven-site-plugin</artifactId>
           <!-- maven-site-plugin is still borked, unless new doxia dependency is explicitly defined. -->
           <!-- latest version of maven-site-plugin causes deploy issues. -->
-          <version>3.7.1</version>
+          <version>3.21.0</version>
           <configuration>
             <!-- we use maven-scm-publish-plugin to deploy, so skip this. -->
             <skipDeploy>true</skipDeploy>