metainfo: fix recommended display length definition

According to https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-relations-display_length, display length's value must be an integer value corresponding to logical pixels.

I took `768`px as value because it corresponds to the recommended referenced size for laptops and tablets, which should match imho the previous incorrect `medium` value.

This fix flathub's build as they don't allow releasing builds with invalid appdata (even if it's just a warning). Running `appstreamcli validate` do raise this warning: `W: org.musicbrainz.Picard:318: relation-display-length-value-invalid medium` currently.

org.musicbrainz.Picard.appdata.xml.in

@@ -76,7 +76,7 @@
   <recommends>
     <control>keyboard</control>
     <control>pointing</control>
-    <display_length compare="ge">medium</display_length>
+    <display_length compare="ge">768</display_length>
     <internet>always</internet>
   </recommends>