Update nightly flatpak runtimes with latest GTK+3 release
Gtk+3 3.22.28 is out, but org.gnome.{Platform,Sdk}//master are still on 3.22.26 (preventing me from building Dconf Editor against them).
I'm reporting this here, because I could not easily find where to report issues with the flatpak runtimes themselves. I hope there is some overlap between both projects, at least enough for someone to point me to the right issue tracker. Thanks for all your efforts!