[GNC] Updated GnuCash with Gnome Runtime v42 available on Flathub

[John Ralls]

This fixes the warning when installing GnuCash from Flathub saying that the GNOME 41 runtime is no longer supported.

The new flatpak is released as version 4.12-2.

Update the org.gnucash.GnuCash flatpak

* via your distribution's package manager or
* by runing 'flatpak --update org.gnucash.GnuCash' on the command line

Regards,
John Ralls