ci: Validate using appstreamcli

[bbhtt]

appstream-util is deprecated since 2020 and Flathub started using the newer appstreamcli tool to validate since 2 weeks ago.

So this can be finally switched.

[bbhtt]

Hey, @lwindolf do you have some brand colour in mind for the app? See https://docs.flathub.org/docs/for-app-authors/metainfo-guidelines/quality-guidelines#brand-colors

I'd also request you to consider updating the screenshots. The current sizes are a bit small for the website and the Yaru theme changed a lot. It would be nice to have a set of fresh screenshots on current GNOME / Ubuntu. I can create some if you want.

[bbhtt]

Something like this maybe:

Light (#CBCFD3):

Dark (#64647D):

[lwindolf]

@bbhtt Thanks for the PR and the hint on the brand color page. Looks interesting!

About updating screenshots. If you could do a set of new ones that would be awesome. I'm too busy with my freelancer work right now I do not find enough time.

[bbhtt]

Ok sure, do you have a preference for Yaru colours or stock GNOME would work too?

[lwindolf]

@bbhtt No preference really.

[bbhtt]

Thanks. I posted some in #1341