Replace custom Purism::form_factor with display_length

See https://github.com/ximion/appstream/issues/476

packaging/moment.metainfo.xml

@@ -23,6 +23,9 @@
   <provides>
     <binary>moment</binary>
   </provides>
+  <requires>
+    <display_length compare="ge">360</display_length>
+  </requires>
   <project_license>LGPL-3.0-or-later</project_license>
   <screenshots>
     <screenshot type="default">
@@ -74,8 +77,4 @@
     <release version="0.4.1" date="2020-03-23"/>
     <release version="0.4.0" date="2020-03-21"/>
   </releases>
-  <custom>
-    <value key="Purism::form_factor">workstation</value>
-    <value key="Purism::form_factor">mobile</value>
-  </custom>
 </component>