Test autoloader without `@` in front of `include_once()` in PHP 8.1

[TimWolla]

https://github.com/WoltLab/WCF/blob/dd19073b48016b8aa4fd299fa676c54b96c8af64/wcfsetup/install/files/lib/system/WCF.class.php#L897

Various classes do not yet have a proper return type, but the deprecation error was suppressed by the @. This should be tested once more without the @ to future-proof the code.