Apply HTML escaping to timestamp attribute in Whitelabel error page

[kwondh5217]

Apply htmlEscape() to the timestamp attribute.

Closes gh-50203

[wilkinsona]

Thanks for the PR, @kwondh5217.

Could you please make similar changes for WebFlux? I think both timestamp and requestId should be escaped there.

Also, please be aware that I've updated the PR's base branch to be 3.5.x as that's the earliest version to which the fix will be applied.

[wilkinsona]

Also, please be aware that I've updated the PR's base branch to be 3.5.x as that's the earliest version to which the fix will be applied.

Please ignore this. I've restored the base branch back to main. Let's get all the changes in place before switching things onto 3.5.x.

[kwondh5217]

Done. Applied htmlEscape() to both timestamp and requestId in AbstractErrorWebExceptionHandler. Also changed the type of timestamp from Date to Object to be consistent with the WebMVC
implementation and to avoid a ClassCastException if a custom ErrorAttributes sets it to a non-Date value. Please take another look !