flatpak: Replace SourceForge links for mirrors

Test data hosted on SourceForge is failing to download way too often and
has been mirrored on gitlab.gnome.org to solve that.

This uses these mirrors instead of the SourceForge links.
4 jobs for wip/aplazas/sourceforge in 21 minutes and 49 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Build
passed flatpak:app #53991

00:14:36

 
  Test
passed flatpak:tests #53992

00:03:25

 
  Review
passed review #53993

00:00:25

passed stop_review #53994
manual

00:03:22