Doc Feedback:Clarify default error response format across API Manager versions

[kavindugeethshan]

Location : https://apim.docs.wso2.com/en/4.0.0/troubleshooting/error-handling/

Description:
The documentation states that the API Manager throws XML based error responses by default in version 4.0.0.
However in newer versions ( 4.6.0) the documentation states that JSON based error responses are returned by default.

While this behavior is correct per version, it can be confusing when comparing documentation across versions without an explicit clarification.

Add a short note indicating that the default error response format is version dependent
(XML in older versions and JSON in newer versions) to improve clarity.

[wso2-engineering-bot]

@wso2-engineering-bot create a PR for this issue following repo conventions