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-project/spring-boot-docs/src/main/java/org/springframework/boot/docs/test/spock/SpockTestRestTemplateExample.java

@@ -46,7 +46,7 @@ public class SpockTestRestTemplateExample {
 				Environment environment) {
 			RestTemplateBuilder builder = builderProvider.getIfAvailable();
 			TestRestTemplate template = builder == null ? new TestRestTemplate()
-					: new TestRestTemplate(builder.build());
+					: new TestRestTemplate(builder);
 			template.setUriTemplateHandler(new LocalHostUriTemplateHandler(environment));
 			return template;
 		}