Support asp.net core 3.1

[raymens]

No description provided.

[mcintyre321]

The build has failed and it doesn't seem that you've updated the example projects. Perhaps you could update it a bit?

[raymens]

The error is due to the CI not having the .NET Core SDK 3.1.
It (should be) both compatible for .NET Core 3.1 apps as well as the "older" .NET Standard apps due to multiple frameworks.
I can add a sample project that uses the new version.

We're already using this in our system by the way as it was a blocking issue for updating to the latest LTS as .NET Core 2.2 is deprecated.

[mcintyre321]

I've just updated Appveyor to use VS 2019, lets see what happens. I would like the .net core example projects in the solution updated before I can merge this though (and please check that the MVC classic one starts up too)