Update web-app version to 3.1

Update the web-app version specified in `web.xml` to 3.1 in order to
make Eclipse happy.

Closes gh-29075

spring-boot-tests/spring-boot-smoke-tests/spring-boot-smoke-test-traditional/src/main/webapp/WEB-INF/web.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<web-app version="2.5" xmlns="http://java.sun.com/xml/ns/javaee"
+<web-app version="3.1" xmlns="http://java.sun.com/xml/ns/javaee"
 	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 	xsi:schemaLocation="http://java.sun.com/xml/ns/javaee https://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
 
@@ -22,7 +22,7 @@
 		<url-pattern>/</url-pattern>
 	</servlet-mapping>
 
-	<!-- Disables Servlet Container welcome file handling. Needed for compatibility 
+	<!-- Disables Servlet Container welcome file handling. Needed for compatibility
 		with Servlet 3.0 and Tomcat 7.0 -->
 	<welcome-file-list>
 		<welcome-file></welcome-file>