Updating generator to for PHP81 compatibility

[funkjedi]

Supress PHP81 deprecation notices which are being thrown for generated classes implementing jsonSerialize.

Deprecated: Return type of ANetApiRequestType::jsonSerialize() should either be compatible with JsonSerializable::jsonSerialize(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice

This updates the generator to add the #[\ReturnTypeWillChange] attribute to the generated jsonSerialize methods.