Disabled use of AppStream for plasma-desktop on stable branches

[Aleix Pol]

Hi,
There is an issue when using an old AppStream version (older than
0.12) with plasma-desktop. It would sometimes crash. Since it's an
optional feature and not central to plasma's usability overall, we
decided to just bump the version, understanding that lacking the
feature is better than crashing.

We recommend to upgrade AppStream to the newer stable release (with
this fix and others) or to just build plasma-desktop without the
AppStream integration if the version of the library is simply too old.

Sorry for the inconvenience.
Aleix

https://bugs.kde.org/show_bug.cgi?id=391468
https://phabricator.kde.org/D15263