Remove TestRestTemplate constructors taking template args

As discussed in gh-11872, `TestRestTemplate` constructor variants taking
a `RestTemplate` argument are confusing since the main goal of that
class is to mutate `RestTemplate`.

This commit removes all those constructor variants and replaces them
with `RestTemplateBuilder` arguments when possible.

Closes gh-11872

spring-boot-samples/spring-boot-sample-actuator-custom-security/src/test/java/sample/actuator/customsecurity/CorsSampleActuatorApplicationTests.java

@@ -27,14 +27,13 @@ import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.test.context.SpringBootTest;
 import org.springframework.boot.test.web.client.LocalHostUriTemplateHandler;
 import org.springframework.boot.test.web.client.TestRestTemplate;
+import org.springframework.boot.web.client.RestTemplateBuilder;
 import org.springframework.context.ApplicationContext;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.RequestEntity;
 import org.springframework.http.ResponseEntity;
-import org.springframework.http.client.HttpComponentsClientHttpRequestFactory;
 import org.springframework.test.context.ActiveProfiles;
 import org.springframework.test.context.junit4.SpringRunner;
-import org.springframework.web.client.RestTemplate;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
@@ -55,12 +54,11 @@ public class CorsSampleActuatorApplicationTests {
 
 	@Before
 	public void setUp() {
-		RestTemplate restTemplate = new RestTemplate();
+		RestTemplateBuilder builder = new RestTemplateBuilder();
 		LocalHostUriTemplateHandler handler = new LocalHostUriTemplateHandler(
 				this.applicationContext.getEnvironment(), "http");
-		restTemplate.setUriTemplateHandler(handler);
-		restTemplate.setRequestFactory(new HttpComponentsClientHttpRequestFactory());
-		this.testRestTemplate = new TestRestTemplate(restTemplate);
+		builder = builder.uriTemplateHandler(handler);
+		this.testRestTemplate = new TestRestTemplate(builder);
 	}
 
 	@Test