flatpak: move to gitlab.gnome.org GMime version
duplicate_mailbox() unit test fails since this commit: https://github.com/jstedfast/gmime/commit/4a80ae527df9aa36fe50fac0878207a31d4d6b72
The commit is ok because it respects RFC 5322: 1 max From/To/Cc/Bcc header.
We can't fix the unit test without breaking Ubuntu/Fedora builds so just use same version.