feat: add vaadin version as CSS classname

Author: javier-godoy

src/main/java/com/flowingcode/vaadin/addons/demo/TabbedDemo.java

@@ -41,6 +41,7 @@
 import com.vaadin.flow.router.Route;
 import com.vaadin.flow.router.RouterLayout;
 import com.vaadin.flow.server.VaadinSession;
+import com.vaadin.flow.server.Version;
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Objects;
@@ -211,6 +212,8 @@ public void showRouterLayoutContent(HasElement content) {
         }
     });
 
+    demo.addClassName("demos-v" + Version.getMajorVersion());
+
     if (helperButton != null) {
       remove(helperButton);
       helperButton = null;