Replace TestRestTemplate with RestTestClient in integration tests

Before deprecating `TestRestTemplate`, we must first stop using it in
integration tests and replace it with:
* `RestTestClient` when the Spring MVC infrastructure is present
* `RestClient` when the test does not have Spring MVC on classpath

See gh-46632

integration-test/spring-boot-test-integration-tests/src/test/java/org/springframework/boot/web/server/test/AbstractSpringBootTestEmbeddedReactiveWebEnvironmentTests.java

@@ -49,6 +49,7 @@ import static org.assertj.core.api.Assertions.assertThat;
  */
 @AutoConfigureWebTestClient
 @AutoConfigureTestRestTemplate
+@SuppressWarnings("removal")
 abstract class AbstractSpringBootTestEmbeddedReactiveWebEnvironmentTests {
 
 	@LocalServerPort

integration-test/spring-boot-test-integration-tests/src/test/java/org/springframework/boot/web/server/test/AbstractSpringBootTestWebServerWebEnvironmentTests.java

@@ -50,6 +50,7 @@ import static org.assertj.core.api.Assertions.assertThat;
  */
 @AutoConfigureRestTestClient
 @AutoConfigureTestRestTemplate
+@SuppressWarnings("removal")
 abstract class AbstractSpringBootTestWebServerWebEnvironmentTests {
 
 	@LocalServerPort