Must use deprecated gtk_show_uri() to display user help correctly in flatpak apps
User help is broken in both Epiphany Tech Preview and Fedora's system Epiphany:
![Screenshot_from_2023-10-04_08-16-31](data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==)
Yelp says:
Document Not Found
The URI ‘help:epiphany/index’ does not point to a valid page.