[8.x] Fail enum validation with pure enums

[goodevilgenius]

The current enum validation (#39437) throws an error if a pure enum is used. Since a pure enum doesn't make sense in this context, the validation should simply fail. This is similar to the behavior for if the validation is used on PHP < 8.1.

Consider the following code:

enum PureEnum
{
    case ONE;
    case TWO;
}

validator(
    ['status' => 'ONE'],
    ['status' => [new Enum(PureEnum::class)],
)->validate();

This code will throw an error, because the code expects a tryFrom static method on the enum. This method only exists on backed enums.

[marcoaureliocardoso]

Enum Validation Rules only works with the BackedEnum Interface
https://laracasts.com/index.php/index.php/discuss/channels/laravel/enum-validation-rules-only-works-by-backedenum-interface