community/phosh-osk-stub: move from testing

Guido gave the green light on switching the default on-screen keyboard for postmarketOS Phosh from Squeekboard to phosh-osk-stub due to the former effectively being unmaintained for soon a year by now. As such, we need to move phosh-osk-stub to community so that it will be available in stable releases as well.

@jane400 Does this look okay to you?