Add UTF8 problem+json media type constant

Issue: SPR-15742

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

@@ -276,6 +276,19 @@ public class MediaType extends MimeType implements Serializable {
 	 * @since 5.0
 	 */
 	public final static String APPLICATION_PROBLEM_JSON_VALUE = "application/problem+json";
+	
+	/**
+	 * Public constant media type for {@code application/problem+json}.
+	 * @since 5.0
+	 * @see <a href="https://tools.ietf.org/html/rfc7807#section-6.1">Problem Details for HTTP APIs, 6.1. application/problem+json</a>
+	 */
+	public final static MediaType APPLICATION_PROBLEM_JSON_UTF8;
+
+	/**
+	 * A String equivalent of {@link MediaType#APPLICATION_PROBLEM_JSON_UTF8}.
+	 * @since 5.0
+	 */
+	public final static String APPLICATION_PROBLEM_JSON_UTF8_VALUE = APPLICATION_PROBLEM_JSON_VALUE + ";charset=UTF-8";
 
 	/**
 	 * Public constant media type for {@code application/problem+xml}.
@@ -302,6 +315,7 @@ public class MediaType extends MimeType implements Serializable {
 		APPLICATION_OCTET_STREAM = valueOf(APPLICATION_OCTET_STREAM_VALUE);
 		APPLICATION_PDF = valueOf(APPLICATION_PDF_VALUE);
 		APPLICATION_PROBLEM_JSON = valueOf(APPLICATION_PROBLEM_JSON_VALUE);
+		APPLICATION_PROBLEM_JSON_UTF8 = valueOf(APPLICATION_PROBLEM_JSON_UTF8_VALUE);
 		APPLICATION_PROBLEM_XML = valueOf(APPLICATION_PROBLEM_XML_VALUE);
 		APPLICATION_RSS_XML = valueOf(APPLICATION_RSS_XML_VALUE);
 		APPLICATION_STREAM_JSON = valueOf(APPLICATION_STREAM_JSON_VALUE);