Update dotnet SDK to to address VS 16.10 regressions

[manodasanW]

• Updated to dotnet 300 series to address pipeline failure after recent VS update
• Removed dependency on older rosyln compilers package as a couple VS version have shipped with the minimum that we require and adding this package forces a downgrade on the compilers package which we use compared to the default one in VS
• Updating versions of source generator package to use release ones from nuget rather than one from dotnet feeds

[Scottj1s]

nice tidy up