PR Summary

The MacOS daily build failed after moving to the latest .NET 6 preview.1 SDK -- dotnet restore was not working.
Remove old .NET SDKs to make 'dotnet restore' works with the latest SDK.

PR Checklist

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• Summarized changes
• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
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• Testing - New and feature
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