Consider to require AppStream 1.0

[probonopd]

https://blog.tenstral.net/2023/11/appstream-1-0-released.html

AppStream has seen lots of changes over the past decade or so, but to their credit they only now are declaring it 1.0 and hence api stable.

Since we don't want a "moving target", we should consider to require AppStream 1.0 in the upcoming versions of the AppImageSpec.

This should hopefully make it easier to validate AppStream data in AppImages in a consistent way.

Wdyt @TheAssassin?