Update .appcast with new tag ✨

Author: marguslant0

.appcast.xml

@@ -1,6 +1,9 @@
 <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
 <rss xmlns:sparkle="http://www.andymatuschak.org/xml-namespaces/sparkle" xmlns:dc="http://purl.org/dc/elements/1.1/" version="2.0">
   <channel>
+    <item>
+      <enclosure url="https://github.com/Frontify/sketch/releases/download/v2.3.21/frontify.sketchplugin.zip" sparkle:version="2.3.21"/>
+    </item>
     <item>
       <enclosure url="https://github.com/Frontify/sketch/releases/download/v2.3.21/frontify.sketchplugin.zip" sparkle:version="2.3.21"/>
     </item>