Ensure .NET versions are installed for testing

Author: mnadareski

.github/workflows/check_pr.yml

@@ -11,7 +11,10 @@ jobs:
     - name: Setup .NET
       uses: actions/setup-dotnet@v4
       with:
-        dotnet-version: 9.0.x
+        dotnet-version: |
+            6.0.x
+            8.0.x
+            9.0.x
       
     - name: Build
       run: dotnet build