run test on pull requests targeting the stable branch

Author: glehmann

.github/workflows/ci.yml

@@ -4,7 +4,9 @@ on:
   push:
     branches: [ "*" ]
   pull_request:
-    branches: [ main ]
+    branches:
+      - main
+      - stable
   workflow_dispatch:
   schedule:
     - cron: '20 7 * * *'