Explicitly set UnsupportedTargetFrameworkVersion for VS support

[ViktorHofer]

Filed dotnet/sdk#44839 to discuss whether UnsupportedTargetFrameworkVersion should be auto-calculated.

This is needed to avoid this error inside VS:

11>------ Build started: Project: System.IO.Pipelines (src\System.IO.Pipelines), Configuration: Debug Any CPU ------
10>C:\Program Files\dotnet\sdk\9.0.100\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.TargetFrameworkInference.targets(171,5): error NETSDK1209: The current Visual Studio version does not support targeting .NET 10.0.  Either target .NET 10.0 or lower, or use Visual Studio version 17.16 or higher
10>Done building project "System.Text.Json.csproj" -- FAILED.

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