desktop: update the appdata to take care of the id change.

From AppStream docs:
«
In previous AppStream releases, the <id/> was used to associate metainfo
files with their .desktop files to merge in data from .desktop files
into the AppStream generator's final output. In modern metainfo files,
the component-ID for desktop-application components can be an arbitrary
string (matching the naming rules applying to all AppStream metadata),
while the <launchable/> tag is used to associate .desktop files with
their metainfo files.
»