Fix AppStream metadata validation
This fixes the following warnings and errors detected by appstreamcli validate org.gnome.Eolie.appdata.xml
:
W - org.gnome.Eolie.appdata.xml:org.gnome.Eolie
The category 'Web' defined is not valid. Refer to the Freedesktop menu
specification for a list of valid categories.
W - org.gnome.Eolie.appdata.xml:org.gnome.Eolie:30
Found empty 'content_rating' tag.
W - org.gnome.Eolie.appdata.xml:org.gnome.Eolie:5
Found invalid tag: 'licence'. Non-standard tags must be prefixed with "x-".
W - org.gnome.Eolie.appdata.xml:org.gnome.Eolie:31
Found invalid tag: 'updatecontact'. Non-standard tags must be prefixed with
"x-".
E - org.gnome.Eolie.appdata.xml:org.gnome.Eolie
The essential tag 'metadata_license' is missing.
E - org.gnome.Eolie.appdata.xml:org.gnome.Eolie:24
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