Update FlatCollection to use the iiif-net package

[sarahos]

This ticket will not contain changes for playwright testing and is more of a tech task

FlatCollection is used to show additional properties to a standard IIIF collection, it would make sense to use inheritance from the centralized iiif-net library collections. This was attempted earlier, but failed due to issues with serialization

see:
https://github.com/digirati-co-uk/iiif-net
https://github.com/dlcs/iiif-presentation/blob/feature/updateCollection/src/IIIFPresentation/Models/API/Collection/FlatCollection.cs

• rename FlatCollection to PresentationCollection
• investigate using inheritance between Collection and FlatCollection
• Possible fix: allow custom serializers in the iiif-net library