Deprecate RestTemplate and related types

As announced in "the state of HTTP clients in Spring" blog post
(https://spring.io/blog/2025/09/30/the-state-of-http-clients-in-spring),
the deprecation timeline for `RestTemplate` was announced last November
and docs were updated accordingly.

This commit `@Deprecate` `RestTemplate` and related types for removal to
send a stronger signal to our community.
The actual removal is scheduled for Spring Framework 8.0 (not yet
scheduled).

Closes gh-36574

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/client/RestTemplateXhrTransport.java

@@ -53,7 +53,10 @@ import org.springframework.web.socket.sockjs.frame.SockJsFrame;
  *
  * @author Rossen Stoyanchev
  * @since 4.1
+ * @deprecated as of 7.1 in favor of {@link RestClientXhrTransport}.
  */
+@Deprecated(since = "7.1", forRemoval = true)
+@SuppressWarnings("removal")
 public class RestTemplateXhrTransport extends AbstractXhrTransport {
 
 	private final RestOperations restTemplate;

spring-websocket/src/test/java/org/springframework/web/socket/sockjs/client/RestTemplateXhrTransportTests.java

@@ -67,6 +67,7 @@ import static org.mockito.Mockito.verifyNoMoreInteractions;
  *
  * @author Rossen Stoyanchev
  */
+@SuppressWarnings("removal")
 class RestTemplateXhrTransportTests {
 
 	private static final JacksonJsonSockJsMessageCodec CODEC = new JacksonJsonSockJsMessageCodec();
@@ -201,6 +202,7 @@ class RestTemplateXhrTransportTests {
 	}
 
 
+	@SuppressWarnings("removal")
 	private static class TestRestTemplate extends RestTemplate {
 
 		private Queue<ClientHttpResponse> responses = new LinkedBlockingDeque<>();