Improve PropertyList<T> and remove beta

Description

Breaking Changes

• EPiServer.Core.Transfer.IRawContentRetriever will no longer populate the Value of each RawProperty. This is instead done by calling the IPropertyExporter.ExportProperties method.
• EPiServer.Core.Transfer.IPropertyImporter has changed slightly to match the new IPropertyExporter interface.
• PropertyData.ToRawString() will no longer be called when exporting data. If a Custom PropertyData type had previously overridden the ToRawValue method, it must now move that functionality to a class that implements IPropertyExportTransform and register it with the Container.
• PropertyJson based properties, such as PropertyList<T>, no longer rely on IObjectSerializer and container registered JsonConverters for its serialization. It now uses Newtonsoft.Json.JsonConvert directly, which means JsonConverters must be defined as attributes on the classes and properties they apply to.