Add trailing slash to dashboard (#5811)

- Boot 3.x changed handling of trailing slash meaning
  it's now adviced to define both /dashboard and
  /dashboard/ in UiController

spring-cloud-dataflow-server-core/src/main/java/org/springframework/cloud/dataflow/server/controller/UiController.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2015-2017 the original author or authors.
+ * Copyright 2015-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -27,7 +27,7 @@
  * @author Gunnar Hillert
  */
 @Controller
-@RequestMapping(UiController.WEB_UI_INDEX_PAGE_ROUTE)
+@RequestMapping({ UiController.WEB_UI_INDEX_PAGE_ROUTE, UiController.WEB_UI_INDEX_PAGE_ROUTE + "/" })
 public class UiController {
 
 	public static final String WEB_UI_INDEX_PAGE_ROUTE = "/dashboard";