SteamOS / Linux - DuckStation update

Hi!

The DuckStation flatpak is dead, the DuckStation dev have stopped supporting it and it has deleted it from the FlatHub store over the weekend.

I've updated EmuDeck so now it now installs the AppImage version.

To upgrade to the AppImage version just open EmuDeck in desktop mode and it will prompt you to migrate DuckStation. No other action is required from your end :)