fixed github dotnet ci

.github/workflows/Adapter CI.yml

@@ -19,7 +19,7 @@ jobs:
     - name: Setup .NET Core
       uses: actions/setup-dotnet@v1
       with:
-        dotnet-version: 2.2.108
+        dotnet-version: 3.1
     - name: Build Adapter
       run: dotnet build
       working-directory: libraries/csharp_dotnetcore/Microsoft.Bot.Builder.Adapters.WeChat