Re-introduce net8 SDK in gh workflow

Author: SonnyRR

.github/workflows/dotnet-buildandtest.yml

@@ -16,7 +16,9 @@ jobs:
     - name: Setup .NET
       uses: actions/setup-dotnet@v4
       with:
-        dotnet-version: 9
+        dotnet-version: |
+          8.x
+          9.x
     - name: Show dotnet version
       run: |
         dotnet --list-sdks