add dotnet 3.1

Author: caroott

.github/workflows/deploy.yml

@@ -12,6 +12,10 @@ jobs:
     steps:
     - name: Checkout
       uses: actions/checkout@v2
+    - name: Setup .NET 3.1
+      uses: actions/setup-dotnet@v1
+      with:
+        dotnet-version: 3.1.301
     - name: Setup .NET 5
       uses: actions/setup-dotnet@v1
       with: