Remove old .NET SDKs to make 'dotnet restore' works with the latest SDK

[daxian-dbw]

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.

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[TravisEz13]

This probably has to be done in the release build as well

[adityapatwardhan]

For some reason release build is fine.

[ghost]

🎉v7.2.0-preview.3 has been released which incorporates this pull request.:tada:

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