Update github build action for gnome 50.

Author: toolstack

.github/workflows/flatpak-builder.yml

@@ -7,7 +7,7 @@ jobs:
     name: "Flatpak"
     runs-on: ubuntu-latest
     container:
-      image: bilelmoussaoui/flatpak-github-actions:gnome-48
+      image: bilelmoussaoui/flatpak-github-actions:gnome-50
       options: --privileged
     steps:
     - uses: actions/checkout@v4