[build] use $(RollForward)=Major for console apps (#778)

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 has only .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 patchx
release, and as `generator.csproj` targets `netcoreapp3.1`, it would
only successfully run on .NET Core 3.1.x.

To change this behavior you can add the `rollForward` value to
`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 editing 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.

Author: jonathanpeppers

Directory.Build.props

@@ -37,6 +37,7 @@
     <TestOutputFullPath>$(MSBuildThisFileDirectory)bin\Test$(Configuration)-$(TargetFramework.ToLowerInvariant())\</TestOutputFullPath>
     <UtilityOutputFullPath Condition=" '$(UtilityOutputFullPathCoreApps)' != '' ">$(UtilityOutputFullPathCoreApps)</UtilityOutputFullPath>
     <UtilityOutputFullPath Condition=" '$(UtilityOutputFullPathCoreApps)' == '' ">$(ToolOutputFullPath)</UtilityOutputFullPath>
+    <RollForward>Major</RollForward>
   </PropertyGroup>
   <PropertyGroup Condition=" '$(JIBuildingForNetCoreApp)' != 'True' ">
     <IntermediateOutputPath>$(BaseIntermediateOutputPath)\$(Configuration)</IntermediateOutputPath>