ci: reduce ci usage by changing workflow rules

Part-of: <https://dev.funkwhale.audio/funkwhale/funkwhale/-/merge_requests/2282>

.gitlab-ci.yml

@@ -17,10 +17,12 @@ default:
 
 workflow:
   rules:
+    # Run for any event on the default branches in the funkwhale namespace
+    - if: >
+        $CI_PROJECT_NAMESPACE == "funkwhale" &&
+        $CI_COMMIT_BRANCH =~ /(stable|develop)/
+    # Run for merge requests from any repo or branches
     - if: $CI_PIPELINE_SOURCE == "merge_request_event"
-    - if: $CI_COMMIT_BRANCH && $CI_OPEN_MERGE_REQUESTS
-      when: never
-    - if: $CI_COMMIT_BRANCH
 
 stages:
   - review