Replace deprecated media type

See gh-17596

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/reactive/error/DefaultErrorWebExceptionHandler.java

@@ -131,7 +131,7 @@ public class DefaultErrorWebExceptionHandler extends AbstractErrorWebExceptionHa
 	protected Mono<ServerResponse> renderErrorResponse(ServerRequest request) {
 		boolean includeStackTrace = isIncludeStackTrace(request, MediaType.ALL);
 		Map<String, Object> error = getErrorAttributes(request, includeStackTrace);
-		return ServerResponse.status(getHttpStatus(error)).contentType(MediaType.APPLICATION_JSON_UTF8)
+		return ServerResponse.status(getHttpStatus(error)).contentType(MediaType.APPLICATION_JSON)
 				.body(BodyInserters.fromObject(error));
 	}