[Feature Request] Add an ASP.NET Core 5.0 target #194

Is your feature request related to a problem? Please describe.
In order for Microsoft.Identity.Web to be referenced by the ASP.NET Core template, we need to support .NET Core 5.0

Describe the solution you'd like

• Just add a target framework in the project.
• From that moment, developer environments and build will need to reference the SDK 5.0.x.
• Update the wiki to express the fact that MIcrosoft.Identity.Web supports .NET 5.0
• Update the project templates

See:

• https://dotnet.microsoft.com/download/dotnet/5.0
• possibly https://dotnet.microsoft.com/download/dotnet-core/scripts for the builds (might not be needed)

To be able to build in Visual Studio enable preview options (Tools | Options | Environment| Preview features | Use previews of the .NET Core SDK (requires restart)

Jenny found this explaining more about .NET 5.0:
https://github.com/dotnet/designs/blob/master/accepted/2020/net5/net5.md