Update flatpak app ID

[albertescanes]

Hi,

Currently, the app ID used to publish on Flathub is io.github.zen_browser.zen. However, the main site has changed to zen-browser.app, and the GitHub page zen-browser.github.io no longer exists.

For this reason, it would be convenient to rename the application ID to something like app.zen_browser.zen.

This means that the application would need to be resubmitted, and an EOL rebase would need to be done on the current repository.

https://docs.flathub.org/docs/for-app-authors/maintenance/#renaming-the-flatpak-id

This way, users would simply receive an update to transition to the new app.

[albertescanes]

Zen has been resubmitted as app.zen_browser.zen. Great!