[Backport 8.x] Run validation on backports too (#2909)

(cherry picked from commit 5d13480)

Co-authored-by: Quentin Pradet <[email protected]>

Author: github-actions[bot]

.github/workflows/validate-pr.yml

@@ -4,6 +4,8 @@ on:
   pull_request:
     branches:
       - main
+      - 8.x
+      - '[0-9]+.[0-9]+'
 
 jobs:
   validate-pr: