Update dotnet.yml

Author: jasonjoh

.github/workflows/dotnet.yml

@@ -16,7 +16,7 @@ jobs:
     - name: Setup .NET
       uses: actions/setup-dotnet@v4
       with:
-        dotnet-version: 8.x
+        dotnet-version: 9.x
     - name: Restore dependencies
       working-directory: user-auth/GraphTutorial/
       run: dotnet restore