Bump up nuget workflow version

Author: macsux

.github/workflows/nuget.yaml

@@ -14,7 +14,7 @@ jobs:
     - name: Setup .NET Core
       uses: actions/setup-dotnet@v1
       with:
-        dotnet-version: 2.2.108
+        dotnet-version: 3.1.200
     - name: test with dotnet
       run: dotnet test
     - name: pack