[build] use $(RollForward) Major for console apps

[jonathanpeppers]

Context: https://natemcmaster.com/blog/2019/01/09/netcore-primitives-3/
Context: https://github.com/dotnet/designs/blob/main/accepted/2019/runtime-binding.md#rollforward

I've seen an error on a CI machine that only has .NET 5 & 6
installed:

Task "BindingsGenerator"
...
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:
    5.0.1 at [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
    6.0.0-alpha.1.20560.10 at [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
You can resolve the problem by installing the specified framework and/or SDK.
- https://aka.ms/dotnet-core-applaunch?framework=Microsoft.NETCore.App&framework_version=3.1.0&arch=x64&rid=osx.10.15-x64
Xamarin.Android.Bindings.ClassParse.targets(35,5): error MSB6006: "dotnet" exited with code 150.

By default, a .NET Core app will only "roll forward" to a patch
release, since generator.csproj targets netcoreapp3.1 it would
only successfully run on .NET Core 3.1.x.

To change this behavior you can modify generator.runtimeconfig.json:

{
  "runtimeOptions": {
    "tfm": "netcoreapp3.1",
    "rollForward": "Major",
    "framework": {
    "name": "Microsoft.NETCore.App",
    "version": "3.1.0"
    }
  }
}

Major appears to be what we want to enable it to pick a newer dotnet
runtime/sdk. This would allow a .NET Core 3.x to be used or some newer
version if no .NET Core 3.x is found:

RollForward can have the following values:

• LatestPatch -- Roll forward to the highest patch version. This
  disables minor version roll forward.
• Minor -- Roll forward to the lowest higher minor version, if
  requested minor version is missing. If the requested minor version
  is present, then the LatestPatch policy is used.
• Major -- Roll forward to lowest higher major version, and lowest
  minor version, if requested major version is missing. If the
  requested major version is present, then the Minor policy is
  used.

We can also use the $(RollForward) MSBuild property as an
alternative to creating the *.runtimeconfig.json file.

I think it's likely best to just use this value across the board, as
there isn't a reason we wouldn't want generator or the other
command-line tools to run on newer .NET versions.

[jpobst]

Looks like something we probably need today, but we also need to be working towards building our stuff with the proper TFM (which we are working towards).

For RTM, we should not be shipping tools built with 3.1 in our 6.0 packages. ;)