Set up .NET 6 in CI builds

Author: nblumhardt

.github/workflows/ci.yml

@@ -26,7 +26,10 @@ jobs:
       - name: Setup
         uses: actions/setup-dotnet@v4
         with:
-          dotnet-version: 9.0.x
+          dotnet-version: |
+            6.0.x
+            8.0.x
+            9.0.x
       - name: Compute build number
         shell: bash
         run: |