Downgrade site plugin again

Bug with markdown generation. Downgrade once again.

pom.xml

@@ -135,7 +135,7 @@
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-site-plugin</artifactId>
-        <version>4.0.0-M16</version>
+        <version>4.0.0-M13</version>
         <configuration>
           <skip>false</skip>  <!-- This is `true` in parent pom -->
           <skipDeploy>false</skipDeploy>  <!-- This is `true` in parent pom -->