PHP8 Compatibility: Annotate return types for jsonSerialize() implementations

[dmf-janw]

Starting with PHP 8, it is required to annotate the return type of jsonSerialize() methods for objects that implement \JsonSerializable.

Without this, one would run into the following fatal error (for example):

PHP Fatal error:  During inheritance of JsonSerializable: Uncaught Exception: Deprecated Functionality: Return type of Doofinder\Management\Model\Item::jsonSerialize() should either be compatible with JsonSerializable::jsonSerialize(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in vendor/doofinder/doofinder/src/Management/Model/Item.php on line 91