Bump org.apache.maven.plugins:maven-site-plugin from 3.12.1 to 3.20.0

Bumps [org.apache.maven.plugins:maven-site-plugin](https://github.com/apache/maven-site-plugin) from 3.12.1 to 3.20.0.
- [Commits](apache/maven-site-plugin@maven-site-plugin-3.12.1...maven-site-plugin-3.20.0)

---
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>

pom.xml

@@ -18,7 +18,7 @@
         <maven-bundle-plugin.version>5.1.9</maven-bundle-plugin.version>
         <maven-resources-plugin.version>3.1.0</maven-resources-plugin.version><!-- for Github CI -->
         <cobertura-maven-plugin.version>2.7</cobertura-maven-plugin.version>
-        <maven-site-plugin.version>3.12.1</maven-site-plugin.version>
+        <maven-site-plugin.version>3.20.0</maven-site-plugin.version>
         <maven-surefire-plugin.version>3.3.1</maven-surefire-plugin.version>
         <jdk9-illegal-access-level>deny</jdk9-illegal-access-level>
     </properties>