Update Flatpak metainfo file

Author: uglide

src/resources/flatpak/dev.rdm.RDM.metainfo.xml

@@ -20,5 +20,7 @@
     <content_rating type="oars-1.0"/>
     <releases>
         <release version="2021.7" date="2021-07-16"/>
+        <release version="2021.8" date="2021-10-26"/>
+        <release version="2021.9" date="2021-11-30"/>
     </releases>
 </component>