Migrate from XPBD to Avian if Avian is ready

[JelaMiraj]

Greetings, as xpbd is being discontinued, I've been migrating my new foxtrot template (recommended to me by a video helping folks get started on the Bevy Game Jam) based app to avian and Oxidized Navigation (& therefore the template too) is the last dependency I have that doesn't have an avian based version. Thanks for putting in the effort to migrate when you do! :D