Upgrade to .NET Core SDK 3.1.201

[xtqqczze]

Support for .NET Core 3.0 ended on 2020-03-03, we should update to 3.1.

[dnfclas]

All CLA requirements met.

[xtqqczze]

OmniSharp.omnisharp-roslyn (Windows):

2020-05-11T04:44:11.1633121Z read: {"MessageType":"TestSession.Message","Payload":{"MessageLevel":2,"Message":"Microsoft.VisualStudio.TestPlatform.ObjectModel.TestPlatformException: Testhost process exited with error: A fatal error occurred, the required library hostfxr.dll could not be found.\r\nIf this is a self-contained application, that library should exist in [C:\Users\VssAdministrator\.nuget\packages\microsoft.testplatform.testhost\16.3.0\build\netcoreapp2.1\x86\].\r\nIf this is a framework-dependent application, install the runtime in the default location [C:\Program Files (x86)\dotnet] or use the DOTNET_ROOT(x86) environment variable to specify the runtime location.\r\n. Please check the diagnostic logs for more information.\r\n at Microsoft.VisualStudio.TestPlatform.CrossPlatEngine.Client.ProxyOperationManager.ThrowExceptionOnConnectionFailure(Int32 connTimeout)\r\n at Microsoft.VisualStudio.TestPlatform.CrossPlatEngine.Client.ProxyOperationManager.SetupChannel(IEnumerable`1 sources, String runSettings)\r\n at Microsoft.VisualStudio.TestPlatform.CrossPlatEngine.Client.ProxyDiscoveryManager.DiscoverTests(DiscoveryCriteria discoveryCriteria, ITestDiscoveryEventsHandler2 eventHandler)"}}

[filipw]

thanks a lot - however there is unfortunately #1782 which was merged today contained the same change already. thanks for your PR.