Remove host compatibility mode for `runtimes.json`

[richlander]

I was just reviewing the RID simplication design. I'm in the process of (finally) merging that spec given that we've been shipping that design for multiple years now. It occured to me that .NET 11 won't have support for most (all?) of the versions in runtimes.json. In the interests of reducing product complexity, I propose we remove this support. It doesn't serve any real purpose at this point. If we choose to remove this support, I'll open a matching issue for NuGet. The same logic applies.

• Simplify RID Model designs#260.
• https://github.com/dotnet/runtime/blob/313982dff3457c8291e8b88cd99785f6920319f0/src/libraries/Microsoft.NETCore.Platforms/src/runtime.json

@elinor-fung

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[agocke]

I'm supportive if it doesn't break anything significant. I agree that it's increasingly bitrotted to the point of uselessness.

[richlander]

Now is a great time to find out.

[richlander]

Would the next step be to change runtimes.json to be just the minimal set? I believe that this file is no longer generated but is still used by the SDK. So is the next step to hand-edit the runtimes.json to match the algorithmic model that the host uses?

@ericstj @nkolev92 @baronfel