Make targetType in UknownContentTypeException transient

Closes gh-31283

spring-web/src/main/java/org/springframework/web/client/UnknownContentTypeException.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -36,8 +36,8 @@ public class UnknownContentTypeException extends RestClientException {
 
 	private static final long serialVersionUID = 2759516676367274084L;
 
-	@SuppressWarnings("serial")
-	private final Type targetType;
+
+	private transient final Type targetType;
 
 	private final MediaType contentType;