Flatpak: Update elementary Platform to 9.1 - #77

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, bilelmoussaoui/flatpak-github-actions/flatpak-builder@v6.4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/