community/gtk4.0: take over maintainership

Currently in postmarketOS we have been forced to fork gtk4 due to some problems in the pinephone. Therefore, it is quite important for pmOS that the GTK4 version stays in-sync with that one in alpine while the correct fix is still in development upstream. Since @Newbyte and me are currently both involved on pmOS and alpine, once tsc#46 (closed) is fixed, this MR shouldn't be needed, as we would probably get notified by the changes here and have time to send an equivalent MR downstream with little delay. However, since that is not yet the case, I request to temporarily take maintenance over this, so that at least one of us gets the notification for updates in GTK4. We could also ask people with merge rights to tag us when updates happen in GTK4, but I assume that is prone to being forgotten. Still, I am not entirely sure that straight taking over maintenance for something like this makes sense, so I thought the best way to do it would be a Draft MR to span some discussion :)