Update flatpak.yml

Author: gavrielsha

.github/workflows/flatpak.yml

@@ -153,7 +153,7 @@ jobs:
           flatpak build-bundle repo fukuro.flatpak com.gavrielsha.Fukuro
       
       - name: Upload Flatpak bundle as artifact
-        uses: actions/upload-artifact@v3
+        uses: actions/upload-artifact@v4
         with:
           name: fukuro-flatpak
           path: fukuro.flatpak