Improvements for PHP 8.1

[SkibidiProduction]

As of PHP 8.1.0, a class which implements Serializable without also implementing __serialize() and __unserialize() will generate a deprecation warning.

Also return type of & Cerbero\Dto\Dto::offsetGet($property) should either be compatible with ArrayAccess::offsetGet(mixed $offset): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice