CI moved to Qt 6.7 for Linux builds

[Kai Uwe Broulik]

Hi,

(only posting to plasma-devel only below is about Plasma)

 > i'd also like to schedule removing CI support for [...] Plasma/5.27

Plasma 5.27 is our LTS release and we’ve just had Kubuntu 24.04 ship 
with it. I’m also aware of at least one other major deployment that’s 
also on 5.27 for the forseeable time.

I think we need to keep this around for at least another year, maybe 
longer, so we can still backport important fixes and do a few more 5.27 
releases. One year of support isn’t exactly “LTS” imho particularly with 
a major version jump. Think of how long Qt 5.15 has been around and 
still will be in the industry.

Cheers
Kai Uwe

PS: Though we should probably disable this stupid appstream test on 5.27