Automatically roll-forward if no .NET Core 3.1 runtime available

[k15tfu]

Hi!

What do you think about adding RollForward Major policy in nunit3-console.csproj to allow it to run on .NET 5 without additional options? Currently I can run it without any problem using .NET Core 3.1 runtime, but on .NET 5 it fails with:

$ .../dotnet-runtime-5.0.9-linux-x64/dotnet exec .../nunit.consolerunner.netcore.3.12.0-beta2/tools/netcoreapp3.1/any/nunit3-console.dll
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:
      5.0.9 at [.../dotnet-runtime-5.0.9-linux-x64/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=ubuntu.18.04-x64

And I need to use --fx-version 5.0.9 or --roll-forward Major options to get it to run successfully. It will also help switching to .NET 6 after release. I can make a PR for this.

Linked issues: #858, #932

cc @ChrisMaddock

[CharliePoole]

@k15tfu I'd like to get this as well as some other bugs fixed for the next release. Your PR would be welcome. Make sure the change is conditioned on the netcoreapp3.1 target. Thanks!

[CharliePoole]

@k15tfu Were you doing a PR for this? I'd like to get it into the 3.14 release

[k15tfu]

@CharliePoole Yes, review please #1093.

[CharliePoole]

@k15tfu Thanks! I merged it. We can talk about your other changes after the release.

[CharliePoole]

This issue has been resolved in version 3.14.0

The release is available on:
GitHub.
NuGet packages are also available NuGet.org and
Chocolatey Packages may be found at Chocolatey.org