Update ci.yml

Author: jamesmcroft

.github/workflows/ci.yml

@@ -53,7 +53,7 @@ jobs:
     - name: Setup .NET
       uses: actions/setup-dotnet@v1
       with:
-        dotnet-version: 3.1.x
+        dotnet-version: 5.x
 
     - name: Setup MSBuild
       uses: microsoft/setup-msbuild@v1.0.2