sync.yml: Add centos9

This will cause our current github action to also sync the
centos9 branch with the latest centos-stream-9 kernel tree

LE-2710

Author: bmastbergen

.github/workflows/sync.yml

@@ -20,6 +20,9 @@ jobs:
            - branch: stable_6.12.y
              remote: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
              remote_branch: linux-6.12.y
+           - branch: centos9
+             remote: https://gitlab.com/redhat/centos-stream/src/kernel/centos-stream-9
+             remote_branch: main
 
     env:
       GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}