Upgrade to .NET Core SDK 3.1.

Author: andykernahan

.github/workflows/dotnet.yaml

@@ -8,7 +8,7 @@ jobs:
         os: [ubuntu-latest, windows-latest, macOS-latest]
     name: Dotnet build
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@master
         with:
           fetch-depth: 0
       - name: Setup dotnet runtime 2.0.9