Rename ServerResponse.BodyBuilder.body to syncBody

This commit renames the `body(Object)` on ServerResponse to
`syncBody(Object)`. The reason for this is that the original method
name clashed with the `body(Publisher)` method in the Kotlin extension.

The new name nicely reflects the synchronous nature of the method,
 making it less appealing than the `Publisher`-based `body` method.

Issue: SPR-15467

spring-test/src/test/java/org/springframework/test/web/reactive/server/samples/JsonContentTests.java

@@ -19,7 +19,6 @@ import java.net.URI;
 
 import org.junit.Test;
 import reactor.core.publisher.Flux;
-import reactor.core.publisher.Mono;
 
 import org.springframework.http.MediaType;
 import org.springframework.http.ResponseEntity;
@@ -64,7 +63,7 @@ public class JsonContentTests {
 	public void postJsonContent() throws Exception {
 		this.client.post().uri("/persons")
 				.contentType(MediaType.APPLICATION_JSON_UTF8)
-				.body("{\"name\":\"John\"}")
+				.syncBody("{\"name\":\"John\"}")
 				.exchange()
 				.expectStatus().isCreated()
 				.expectBody().isEmpty();

spring-test/src/test/java/org/springframework/test/web/reactive/server/samples/ResponseEntityTests.java

@@ -123,7 +123,7 @@ public class ResponseEntityTests {
 	@Test
 	public void postEntity() throws Exception {
 		this.client.post().uri("/persons")
-				.body(new Person("John"))
+				.syncBody(new Person("John"))
 				.exchange()
 				.expectStatus().isCreated()
 				.expectHeader().valueEquals("location", "/persons/John")