change dotnet version

Author: tocsoft

.github/workflows/dotnet.yml

@@ -19,7 +19,7 @@ jobs:
     - name: Setup .NET
       uses: actions/setup-dotnet@v1
       with:
-        dotnet-version: 5.0.x
+        dotnet-version: 3.1.x
     - name: Restore dependencies
       run: dotnet restore
     - name: Build