Bug 1621636 - enable Flatpak push to stable for m-r. r=rail

Differential Revision: https://phabricator.services.mozilla.com/D66395

--HG--
extra : moz-landing-system : lando

taskcluster/ci/release-flatpak-push/kind.yml

@@ -29,7 +29,7 @@ job-defaults:
         channel:
             by-release-type:
                 beta: beta
-                release: mock  # TODO: once we are ready to ship to stable, update this
+                release: stable
                 default: mock
 
 jobs: