Make HttpStatusCode extend Serializable

`HttpStatusCode` instance only hold an int value and are held by
`Serializable` classes, so this commit enforces this.

Without this change, Java 19+ will emit a compiler warning as
`Serializable` classes use `HttpStatusCode` as a field.

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

@@ -16,6 +16,8 @@
 
 package org.springframework.http;
 
+import java.io.Serializable;
+
 import org.springframework.util.Assert;
 
 /**
@@ -27,7 +29,7 @@ import org.springframework.util.Assert;
  * @see <a href="https://www.iana.org/assignments/http-status-codes">HTTP Status Code Registry</a>
  * @see <a href="https://en.wikipedia.org/wiki/List_of_HTTP_status_codes">List of HTTP status codes - Wikipedia</a>
  */
-public sealed interface HttpStatusCode permits DefaultHttpStatusCode, HttpStatus {
+public sealed interface HttpStatusCode extends Serializable permits DefaultHttpStatusCode, HttpStatus {
 
 	/**
 	 * Return the integer value of this status code.