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-web/src/test/java/org/springframework/http/client/support/InterceptingHttpAccessorTests.java

@@ -35,6 +35,7 @@ import static org.assertj.core.api.Assertions.assertThat;
  *
  * @author Brian Clozel
  */
+@SuppressWarnings("removal")
 class InterceptingHttpAccessorTests {
 
 	@Test

spring-web/src/test/java/org/springframework/web/client/DefaultResponseErrorHandlerHttpStatusTests.java

@@ -57,6 +57,7 @@ import static org.springframework.http.HttpStatus.UNPROCESSABLE_ENTITY;
  * Tests for {@link DefaultResponseErrorHandler} handling of specific
  * HTTP status codes.
  */
+@SuppressWarnings("removal")
 class DefaultResponseErrorHandlerHttpStatusTests {
 
 	private final DefaultResponseErrorHandler handler = new DefaultResponseErrorHandler();

spring-web/src/test/java/org/springframework/web/client/DefaultResponseErrorHandlerTests.java

@@ -44,6 +44,7 @@ import static org.mockito.Mockito.mock;
  * @author Juergen Hoeller
  * @author Denys Ivano
  */
+@SuppressWarnings("removal")
 class DefaultResponseErrorHandlerTests {
 
 	private final DefaultResponseErrorHandler handler = new DefaultResponseErrorHandler();

spring-web/src/test/java/org/springframework/web/client/ExtractingResponseErrorHandlerTests.java

@@ -44,7 +44,7 @@ import static org.mockito.Mockito.mock;
  *
  * @author Arjen Poutsma
  */
-@SuppressWarnings("ALL")
+@SuppressWarnings("removal")
 class ExtractingResponseErrorHandlerTests {
 
 	private ExtractingResponseErrorHandler errorHandler;

spring-web/src/test/java/org/springframework/web/client/HttpMessageConverterExtractorTests.java

@@ -52,6 +52,7 @@ import static org.mockito.Mockito.mock;
  * @author Brian Clozel
  * @author Sam Brannen
  */
+@SuppressWarnings("removal")
 class HttpMessageConverterExtractorTests {
 
 	private final HttpMessageConverter<String> converter = mock();

spring-web/src/test/java/org/springframework/web/client/RestClientBuilderTests.java

@@ -49,7 +49,7 @@ import static org.assertj.core.api.Assertions.fail;
  */
 class RestClientBuilderTests {
 
-	@SuppressWarnings("unchecked")
+	@SuppressWarnings({"unchecked", "removal"})
 	@Test
 	void createFromRestTemplate() {
 		JettyClientHttpRequestFactory requestFactory = new JettyClientHttpRequestFactory();
@@ -90,6 +90,7 @@ class RestClientBuilderTests {
 	}
 
 	@Test
+	@SuppressWarnings("removal")
 	void defaultUriBuilderFactory() {
 		RestTemplate restTemplate = new RestTemplate();
 

spring-web/src/test/java/org/springframework/web/client/RestTemplateIntegrationTests.java

@@ -82,6 +82,7 @@ import static org.springframework.http.MediaType.MULTIPART_MIXED;
  * @author Brian Clozel
  * @author Sam Brannen
  */
+@SuppressWarnings("removal")
 class RestTemplateIntegrationTests extends AbstractMockWebServerTests {
 
 	@Retention(RetentionPolicy.RUNTIME)

spring-web/src/test/java/org/springframework/web/client/RestTemplateObservationTests.java

@@ -55,6 +55,7 @@ import static org.springframework.http.HttpMethod.GET;
  * Tests for the client HTTP observations with {@link RestTemplate}.
  * @author Brian Clozel
  */
+@SuppressWarnings("removal")
 class RestTemplateObservationTests {
 
 

spring-web/src/test/java/org/springframework/web/client/RestTemplateTests.java

@@ -83,7 +83,7 @@ import static org.springframework.http.MediaType.parseMediaType;
  * @author Brian Clozel
  * @author Sam Brannen
  */
-@SuppressWarnings("unchecked")
+@SuppressWarnings({"unchecked", "removal"})
 class RestTemplateTests {
 
 	private final ClientHttpRequestFactory requestFactory = mock();

spring-web/src/test/java/org/springframework/web/client/support/RestClientAdapterTests.java

@@ -80,7 +80,7 @@ import static org.assertj.core.api.Assertions.assertThatThrownBy;
  * @author Rossen Stoyanchev
  * @author Brian Clozel
  */
-@SuppressWarnings("JUnitMalformedDeclaration")
+@SuppressWarnings({"JUnitMalformedDeclaration", "removal"})
 class RestClientAdapterTests {
 
 	private final MockWebServer anotherServer = new MockWebServer();