Upgrade maven-site-plugin 3.12.1 -> 3.20.0 (#1296)

See:
- https://issues.apache.org/jira/issues/?jql=project%20%3D%20MSITE%20AND%20fixVersion%20%3E%203.12.1%20AND%20fixVersion%20%3C%3D%203.20.0
- https://github.com/apache/maven-site-plugin/releases/tag/maven-site-plugin-3.20.0
- apache/maven-site-plugin@maven-site-plugin-3.12.1...maven-site-plugin-3.20.0

pom.xml

@@ -1244,7 +1244,7 @@
                 <plugin>
                     <groupId>org.apache.maven.plugins</groupId>
                     <artifactId>maven-site-plugin</artifactId>
-                    <version>3.12.1</version>
+                    <version>3.20.0</version>
                 </plugin>
                 <plugin>
                     <groupId>org.apache.maven.plugins</groupId>