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">