Fix APPLICATION_PROBLEM_JSON_UTF8 media type value

This commit adds the missing "+json" suffix to the "application/problem"
media type for its deprecated UTF-8 variant.

Fixes gh-23825

Author: bclozel

spring-web/src/main/java/org/springframework/http/MediaType.java

@@ -370,7 +370,7 @@ public class MediaType extends MimeType implements Serializable {
 		APPLICATION_OCTET_STREAM = new MediaType("application", "octet-stream");
 		APPLICATION_PDF = new MediaType("application", "pdf");
 		APPLICATION_PROBLEM_JSON = new MediaType("application", "problem+json");
-		APPLICATION_PROBLEM_JSON_UTF8 = new MediaType("application", "problem", StandardCharsets.UTF_8);
+		APPLICATION_PROBLEM_JSON_UTF8 = new MediaType("application", "problem+json", StandardCharsets.UTF_8);
 		APPLICATION_PROBLEM_XML = new MediaType("application", "problem+xml");
 		APPLICATION_RSS_XML = new MediaType("application", "rss+xml");
 		APPLICATION_STREAM_JSON = new MediaType("application", "stream+json");