ci: fix the branch patterns of the 'Check cherry-picks' action

For reference on the acceptable pattern:
https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#filter-pattern-cheat-sheet

.github/workflows/check-cherry-picks.yml

@@ -2,8 +2,8 @@ name: "Check cherry-picks"
 on:
   pull_request_target:
     branches:
-     - 'release-*'
-     - 'staging-*'
+     - 'release-**'
+     - 'staging-**'
 
 permissions: {}