Change dotnet-version to the latest one (#17)

Author: ogun

.github/workflows/dotnetcore.yml

@@ -12,7 +12,7 @@ jobs:
       - name: Setup .NET Core
         uses: actions/setup-dotnet@v1
         with:
-          dotnet-version: 3.1.100
+          dotnet-version: 3.1.x
       - name: Run unit tests
         run: dotnet test --configuration Release ImageServer.Tests/
       - name: Run integration tests