TEMP: hack until .NET 8 is shipped and/or .NET 9 SDK is coherent

[RussKie]

I tested it locally:

dotnet build
dotnet tool uninstall --global Microsoft.VisualStudio.SlnGen.Tool
dotnet tool install --global Microsoft.VisualStudio.SlnGen.Tool --add-source D:\Development\microsoft-slngen\src\Microsoft.VisualStudio.SlnGen.Tool\bin\Debug --prerelease

Before:

PS D:\Development\dotnet-extensions> .\restore.cmd -vs * -nolaunch

SlnGen does not currently support the .NET SDK 9.0.100-alpha.1.23464.17 defined by in global.json.  Please update to the latest version and if you still get this error message, file an issue at https://github.com/microsoft/slngen/issues/new so it can be added.
Failed to generate the solution.

After:

PS D:\Development\dotnet-extensions> .\restore.cmd -vs * -nolaunch

Build started 25/09/2023 10:58:46 AM.
Generating solution for project "D:\Development\dotnet-extensions\test\TestUtilities\TestUtilities.csproj"
Generating solution for project "D:\Development\dotnet-extensions\src\Analyzers\Microsoft.Analyzers.Extra\Microsoft.Analyzers.Extra.csproj"
Generating solution for project "D:\Development\dotnet-extensions\src\Analyzers\Microsoft.Analyzers.Local\Microsoft.Analyzers.Local.csproj"
Generating solution for project "D:\Development\dotnet-extensions\src\Generators\Microsoft.Gen.AutoClient\Microsoft.Gen.AutoClient.csproj"
....

[jeffkl]

Thanks for the contribution!