Backporting AppStream changes

[Carl Schwan]

Hi,

I would like to backport a bunch of AppStream changes to the 24.12 branch of some gear apps:

- https://invent.kde.org/pim/kleopatra/-/merge_requests/331/
- https://invent.kde.org/pim/kleopatra/-/merge_requests/330 (the text for that is exactly the same used on apps-kde-org so hopefully part of the translation memory already)
- https://invent.kde.org/utilities/kate/-/merge_requests/1662
- https://invent.kde.org/graphics/arianna/-/commit/df1a644d1ea8d6761324be43c455750c196c5255
- https://invent.kde.org/network/kget/-/commit/c6e596d814f40142805e7634eb38a51e428b91af
- And probably a bunch more of these sort of small changes for other apps

The goal is to polish a bit our application listing on Flathub (and other stores) on time for Gear 24.12 so what we don't have to wait for 25.04.

apps.kde.org uses the strings from master, so this has no impact there. Flathub doesn't seems to show any translations. So the only places where this has an impact is in Discover/Gnome Center where this information is displayed.

Cheers,
Carl