Update flatpak runtime to 5.15-22.08

This seems to work fine apart from that it does cause a regression where the cursor no longer follows my theme and instead is some stock cursor. Is this a known issue with the 5.15-22.08 runtime? I can't see how this would be Basket-specific. Marking as draft until this has been reconciled.

For what it's worth, I'm on GNOME.

I cannot reproduce the aforementioned issue any more. I've been using Basket with the 22.08 runtime for a few months now and it's working just fine for me. I think this is ready to be merged.