Upgrade to .NET9 | GitHub Action

Author: AlexCatarino

.github/workflows/url_check_workflow.yml

@@ -16,7 +16,7 @@ jobs:
       - name: Setup dotnet
         uses: actions/setup-dotnet@v3
         with:
-          dotnet-version: '6.0.x'
+          dotnet-version: '9.0.x'
 
       - name: Build and run the url tester csproj
         run: |