GitHubActionsCI: switch to .NET6 to fix build

Author: webwarrior-ws

.github/workflows/CI.yml

@@ -15,7 +15,7 @@ jobs:
     - name: Setup .NET
       uses: actions/setup-dotnet@v3
       with:
-        dotnet-version: 8.0.415
+        dotnet-version: 6.0.428
     - name: Build & Run Tests
       run: |
         .\build.cmd