Use current dotnet versions

Author: davidjgoss

.github/workflows/test-dotnet.yml

@@ -18,7 +18,9 @@ jobs:
       - name: Setup .NET
         uses: actions/setup-dotnet@v4
         with:
-          dotnet-version: 6.0.x
+          dotnet-version: |
+            8.0.x
+            9.0.x
       - name: Restore dependencies
         run: dotnet restore
         working-directory: dotnet