(GH-3455) Update Microsoft.NETCore.Platforms to 6.0.0-preview.7.21377.19
(GH-3455) Update Microsoft.NETCore.Platforms to 6.0.0-preview.7.21377.19

* fixes #3455

cake-build/cake,patriksvensson/cake,cake-build/cake,devlead/cake,patriksvensson/cake,gep13/cake,gep13/cake,devlead/cake