Upgrade to .NET 5.0

[ManickaP]

Could the cli tool runfo be updated to work with .NET 5.0?

I don't have 3.1 on my machine and it fails with:

It was not possible to find any compatible framework version
The framework 'Microsoft.NETCore.App', version '3.1.0' was not found.
  - The following frameworks were found:
      2.1.24 at [/usr/share/dotnet/shared/Microsoft.NETCore.App]
      5.0.0 at [/usr/share/dotnet/shared/Microsoft.NETCore.App]
      5.0.2 at [/usr/share/dotnet/shared/Microsoft.NETCore.App]

You can resolve the problem by installing the specified framework and/or SDK.

The specified framework can be found at:
  - https://aka.ms/dotnet-core-applaunch?framework=Microsoft.NETCore.App&framework_version=3.1.0&arch=x64&rid=manjaro-x64

[akoeplinger]

We should set the <RollForward> option in the runfo.csproj so it uses later versions automatically.

[jaredpar]

Think rolling forward makes sense here.

I'd like to move to net5.0 but having to wait till some of the Azure Infra moves there so I can just move the entire solution to net5.0.