Consider removing the win-arm RID from the RID graph

[baronfel]

Today we expose the win-arm RID in the portable RID graph, which allows users to target that RID, as well as author and distribute assets for that RID. However, we don't ship an AppHost (and likely other critical assets) for this RID, so users that attempt to target it have a poor experience. Here's an example:

E:\..\..\win-arm-demo> dotnet new console
The template "Console App" was created successfully.

Processing post-creation actions...
Restoring E:\Code\Scratch\win-arm-demo\win-arm-demo.csproj:
Restore succeeded.

E:\..\..\win-arm-demo> dotnet build --arch arm --os win
C:\Program Files\dotnet\sdk\9.0.200-preview.0.24575.35\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.Sdk.FrameworkReferenceResolution.targets(158,5): error NETSDK1084: There is no application host available for the specified RuntimeIdentifier 'win-arm'.
C:\Program Files\dotnet\sdk\9.0.200-preview.0.24575.35\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.Sdk.FrameworkReferenceResolution.targets(158,5): error NETSDK1084: There is no application host available for the specified RuntimeIdentifier 'win-arm'.
C:\Program Files\dotnet\sdk\9.0.200-preview.0.24575.35\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.Sdk.FrameworkReferenceResolution.targets(158,5): error NETSDK1084: There is no application host available for the specified RuntimeIdentifier 'win-arm'.
C:\Program Files\dotnet\sdk\9.0.200-preview.0.24575.35\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.Sdk.FrameworkReferenceResolution.targets(158,5): error NETSDK1084: There is no application host available for the specified RuntimeIdentifier 'win-arm'.

Restore failed with 4 error(s) in 0.4s

E:\..\..\win-arm-demo> dotnet build --arch arm --os win -p:UseAppHost=false
Restore complete (0.3s)
You are using a preview version of .NET. See: https://aka.ms/dotnet-support-policy
  win-arm-demo succeeded (2.1s) → bin\Debug\net9.0\win-arm\win-arm-demo.dll

Build succeeded in 2.8s

Do we want to allow targeting this old RID? A while back we said that we dropped support for it.

[dotnet-policy-service]

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