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-test/src/test/java/org/springframework/test/web/client/samples/MockMvcClientHttpRequestFactoryTests.java

@@ -47,7 +47,7 @@ import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
  * @author Juergen Hoeller
  */
 @SpringJUnitWebConfig
-@SuppressWarnings("deprecation")
+@SuppressWarnings({"deprecation", "removal"})
 class MockMvcClientHttpRequestFactoryTests {
 
 	private final RestTemplate template;