[v21.12] main/postmarketos-upgrade-gtk3: new aport

Add the updated postmarketos-upgrade-gtk3 app as dependency of postmarketos-ui-phosh (so it gets installed on existing installs).

For more details see https://gitlab.com/postmarketOS/postmarketos-upgrade-gtk3/-/merge_requests/3

WIP only because the release is not tagged yet (needs testing), and should only be merged once v21.12 is EOL (or at least definitely only after the v22.06 release)