Upgrade to work with Episerver Commerce 14

[stefanolsen]

Even though this add-on is made for .NET Framework 4.8, it sort of works on .NET 5. But it depends on, and imports, several .NET FX packages that should not be used in a .NET 5 application:

• EntityFramework (should be replaced with EntityFramework Core)
• Newtonsoft.Json (can still work, but nice to replace with System.Text.Json)
• System.Configuration
• System.Web.Http
• (Etc.)

It would also be nice to split up the add-in into two separate packages. For instance: Vipps.Payment.Client (with only the API integration) and Vipps.Payment (for the payment gateway, API controllers and polling logic). This is in true spirit of the new Episerver package structure and it makes it possible for partner integrators to pick and choose parts of the integration, as needed.