Package as Flatpak for elementary OS 6

[danirabbit]

As you might have heard, AppCenter is going Flatpak! For elementary OS 6, we will not longer be accepting Debian packaging, so your app will need to be updated.

We have documentation on our developer guide for writing your Flatpak manifest here: https://docs.elementary.io/develop/writing-apps/our-first-app/packaging#flatpak-manifest

Also, in case you missed it, elementary OS 6 beta 2 is now available. This is a great time to get your app packaged up as Flatpak and test that everything is working before the final release. AppCenter submission for OS 6 will be open soon, so don't delay :)