Bump org.apache.maven.plugins:maven-site-plugin from 3.20.0 to 3.21.0

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
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

pom.xml

@@ -29,7 +29,7 @@
         <jxr.version>3.5.0</jxr.version>
         <taglist.version>3.2.1</taglist.version>
         <spotbugs.version>4.8.6.4</spotbugs.version>
-        <m3.site.version>3.20.0</m3.site.version>
+        <m3.site.version>3.21.0</m3.site.version>
         <changelog.version>2.2</changelog.version>
         <changelog.version>2.2</changelog.version>
         <jacoco.version>0.8.12</jacoco.version>