Do not use application/graphql as default MIME type

As seen in spring-projects/spring-graphql#375, Spring Boot should also
use "application/json" as the default MIME type but remain compatible
with "application/graphql+json" still if clients POST content with this
type or explicitly accept it.

Closes gh-30860

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/graphql/rsocket/RSocketGraphQlClientAutoConfiguration.java

@@ -51,7 +51,7 @@ public class RSocketGraphQlClientAutoConfiguration {
 	@ConditionalOnMissingBean
 	public RSocketGraphQlClient.Builder<?> rsocketGraphQlClientBuilder(
 			RSocketRequester.Builder rsocketRequesterBuilder) {
-		return RSocketGraphQlClient.builder(rsocketRequesterBuilder.dataMimeType(MimeTypeUtils.APPLICATION_GRAPHQL));
+		return RSocketGraphQlClient.builder(rsocketRequesterBuilder.dataMimeType(MimeTypeUtils.APPLICATION_JSON));
 	}
 
 }

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/graphql/reactive/GraphQlWebFluxAutoConfigurationTests.java

@@ -75,7 +75,7 @@ void simpleQueryShouldWork() {
 		testWithWebClient((client) -> {
 			String query = "{ bookById(id: \\\"book-1\\\"){ id name pageCount author } }";
 			client.post().uri("/graphql").bodyValue("{  \"query\": \"" + query + "\"}").exchange().expectStatus().isOk()
-					.expectHeader().contentType("application/graphql+json").expectBody().jsonPath("data.bookById.name")
+					.expectHeader().contentType("application/json").expectBody().jsonPath("data.bookById.name")
 					.isEqualTo("GraphQL for beginners");
 		});
 	}
@@ -151,8 +151,8 @@ private void testWithWebClient(Consumer<WebTestClient> consumer) {
 		this.contextRunner.run((context) -> {
 			WebTestClient client = WebTestClient.bindToApplicationContext(context).configureClient()
 					.defaultHeaders((headers) -> {
-						headers.setContentType(MediaType.APPLICATION_GRAPHQL);
-						headers.setAccept(Collections.singletonList(MediaType.APPLICATION_GRAPHQL));
+						headers.setContentType(MediaType.APPLICATION_JSON);
+						headers.setAccept(Collections.singletonList(MediaType.APPLICATION_JSON));
 					}).baseUrl(BASE_URL).build();
 			consumer.accept(client);
 		});

spring-boot-project/spring-boot-test/src/test/java/org/springframework/boot/test/graphql/tester/HttpGraphQlTesterContextCustomizerIntegrationTests.java

@@ -79,7 +79,7 @@ static class TestHandler implements HttpHandler {
 		@Override
 		public Mono<Void> handle(ServerHttpRequest request, ServerHttpResponse response) {
 			response.setStatusCode(HttpStatus.OK);
-			response.getHeaders().setContentType(MediaType.APPLICATION_GRAPHQL);
+			response.getHeaders().setContentType(MediaType.APPLICATION_JSON);
 			return response.writeWith(Mono.just(factory.wrap("{\"data\":{}}".getBytes())));
 		}
 

spring-boot-project/spring-boot-test/src/test/java/org/springframework/boot/test/graphql/tester/HttpGraphQlTesterContextCustomizerWithCustomBasePathTests.java

@@ -79,7 +79,7 @@ static class TestHandler implements HttpHandler {
 		@Override
 		public Mono<Void> handle(ServerHttpRequest request, ServerHttpResponse response) {
 			response.setStatusCode(HttpStatus.OK);
-			response.getHeaders().setContentType(MediaType.APPLICATION_GRAPHQL);
+			response.getHeaders().setContentType(MediaType.APPLICATION_JSON);
 			return response.writeWith(Mono.just(factory.wrap("{\"data\":{}}".getBytes())));
 		}
 

spring-boot-project/spring-boot-test/src/test/java/org/springframework/boot/test/graphql/tester/HttpGraphQlTesterContextCustomizerWithCustomContextPathTests.java

@@ -70,7 +70,7 @@ DispatcherServlet dispatcherServlet() {
 	@RestController
 	static class TestController {
 
-		@PostMapping(path = "/graphql", produces = MediaType.APPLICATION_GRAPHQL_VALUE)
+		@PostMapping(path = "/graphql", produces = MediaType.APPLICATION_JSON_VALUE)
 		String graphql() {
 			return "{}";
 		}