YARN-8401. [UI2] new ui is not accessible with out internet connection. Contributed by Bibin A Chundatt.

(cherry picked from commit fbaff369e9b9022723a7b2c6f25e71122a8f8a15)

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-ui/src/main/webapp/WEB-INF/web.xml

@@ -16,10 +16,6 @@
 * limitations under the License.
 -->
 
-<!DOCTYPE web-app PUBLIC
- "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
- "http://java.sun.com/dtd/web-app_2_3.dtd" >
-
-<web-app>
+<web-app version="2.4" xmlns="http://java.sun.com/xml/ns/j2ee">
   <display-name>YARN UI</display-name>
 </web-app>