Increase the metadata size limit to 128MB

Increase the metadata size limit to 128MB

Flathub has hit the 10MB limit in 2022, and we had to drop less popular
CPU architectures from the main summary to subsummaries, effectively
cutting off users running too old Flatpak version. Despite that, the
main summary containing only x86_64 is already at 7MB. As this is
eventually going to happen to subsummaries as well, preemptively bump
the limit 12 times.

It takes between 2 and 3 years for a change like this to roll out across
Linux distributions so the best time for this was yesterday.

fixes #2715