Update URLs to GitLab in Flatpak manifest
The GitLab migration happened and all the modules that previously were pointed to git.gnome.org now should point to gitlab.gnome.org.
This fixes the Flatpak nightly build.
The GitLab migration happened and all the modules that previously were pointed to git.gnome.org now should point to gitlab.gnome.org.
This fixes the Flatpak nightly build.