build(deps): bump org.apache.maven.plugins:maven-site-plugin (#1888)

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

pom.xml

@@ -1217,7 +1217,7 @@
             It also seems that as soon as we update to 3.20.0 (the next version) the asciidoctor plugin stops
             generating output.
           -->
-          <version>3.12.1</version>
+          <version>3.21.0</version>
 
           <configuration>
             <!--templateFile>${session.executionRootDirectory}/src/site/template/site.vm</templateFile-->