Make ProblemDetail implement Serializable

[takashno]

Expected Behavior

The ProblemDetail class should be serializable
so that it can be used in distributed environments where serialization is required.

Actual Behavior

Currently, ProblemDetail does not implement Serializable,
which can cause issues when attempting to serialize instances of this class.

Suggested Fix

Modify the class declaration as follows:

Before

public class ProblemDetail {

After

import java.io.Serializable

public class ProblemDetail implements Serializable {

Use Case

In some scenarios, such as caching or distributed systems, ProblemDetail instances need to be serialized.
Making it Serializable would improve compatibility with frameworks that rely on serialization.

Environment

Spring Framework Version: v6.2.1
JDK Version: corretto 21

Additional Information

If there is a specific reason why ProblemDetail was not made Serializable,
it would be helpful to clarify whether this is intentional or an oversight.