.net tool: clarify install instructions

Because we use dotnet publish (which requires a specific framework), to
create the payload of our .NET tool package, the package is tied to the
framework specified. This means that it will only work with the
corresponding .NET SDK for that framework.

In light of this, update the .NET tool install instructions to specify
that users must download the latest .NET 6.0 SDK as a prerequisite for
installing the package, since that is the framework with which we
currently published. This will also need to be updated whenever we upgrade
to a new Target Framework.

Additionally, remove Windows as a supported .NET tool platform. Since we
currently target .NET Framework rather than .NET, the package cannot
actually be installed on Windows.

Author: ldennington

docs/install.md

@@ -204,10 +204,11 @@ tool][dotnet-tool]. This is
 the preferred install method for Linux because you can use it to install on any
 [.NET-supported
 distribution][dotnet-supported-distributions]. You
-can also use this method on macOS or Windows if you so choose.
+can also use this method on macOS if you so choose.
 
-**Note:** Make sure you have installed .NET before attempting to run the
-following `dotnet tool` commands.
+**Note:** Make sure you have installed the [latest version of the .NET 6.0
+SDK](https://dotnet.microsoft.com/en-us/download/dotnet/6.0) before attempting
+to run the following `dotnet tool` commands.
 
 #### Install