Recommend Bazaar instead of Software for Ubuntu

[AsciiWolf]

We should consider recommending Bazaar instead of GNOME Software in the Ubuntu setup instructions. GNOME Software in Ubuntu also handles classic deb packages (including update notifications and mime type handling for local debs) and interferes with the default App Center.

Bazaar on the other hand is focused on Flatpaks and provides a much better user experience.

[AsciiWolf]

We could also recommend it for other distros that have no GUI support for managing Flatpak apps.