Update flatpack metadata to v2.1.0.

Author: bladecoder

adventure-editor/desktop/com.bladecoder.adventure-editor.appdata.xml

@@ -18,7 +18,7 @@
     </screenshot>
   </screenshots>
   <releases>
-    <release version="2.0.0" date="2018-01-22"/>
+    <release version="2.1.0" date="2018-07-03"/>
   </releases>
   <update_contact>[email protected]</update_contact>