.NET 5 Support

[vse3211]

Please, update this project to .NET 5

"IApplicationBuilder" does not contain a definition for "UsePayPalAuthentication", and the most appropriate overloaded extension method "PayPalAuthenticationExtensions.UsePayPalAuthentication(IAppBuilder, PayPalAuthenticationOptions)"

[MingweiSamuel]

https://github.com/aspnet-contrib/AspNet.Security.OAuth.Providers