[ristretto] Convert Appdata file to Appstream

Submitted by Yves-Alexis Perez

Assigned to Stephan Arts

Link to original bug (#14620)

Description

Hi,

similar to #14037 and #14094 ristretto ships outdated Appdata file (in /usr/share/appdata/ristretto.appdata.xml). This should be converted to Appstream format and stored in /usr/share/metainfo/. appstream-util validate can be used to test the file