Update screenshots on /doc/get/verify_the_iso_image_using_gnome

The new ones don’t have logo, that’s expected in GNOME 3 and would have to be fixed in seahorse-nautilus.

But that’s also blocked by #7249 (closed): Truncated notifications.

Feature Branch: doc/7259-seahorse-nautilus-screenshots

Original created by @sajolida on 7259 (Redmine)

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