Source Archive switched from TuxFamily to GitHub Releases

Since the TuxFamily upload server has been very unreliable and down for several days, we have now switched to GitHub releases as hosting service for the source archive (see issue #2799).

The format of the source archive URL now looks like: https://github.com/pbek/QOwnNotes/releases/download/v23.7.0/qownnotes-23.7.0.tar.xz