Upgrade .NET version in GitHub Actions workflow to 9.0.x

Author: elbruno

.github/workflows/dotnet-build.yml

@@ -16,7 +16,7 @@ jobs:
     - name: Setup .NET
       uses: actions/setup-dotnet@v3
       with:
-        dotnet-version: 8.0.x
+        dotnet-version: 9.0.x
     - name: Restore dependencies
       run: dotnet restore ./app/app.sln
     - name: Build