Upgrade csharpcore from .NET Core 2.1 to 3.1
Upgrade csharpcore from .NET Core 2.1 to 3.1

.NET Core 2.1 loses support on the 21st August 2021. The current Long
Term Support version of .NET Core is 3.1. See https://docs.microsoft.com/en-us/lifecycle/products/microsoft-net-and-net-core for details.

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