doc: add `backport-open-vN.x` step to backporting guide
Backporting guide skipped a step about backport-open
label between backport-requested
and backported-to
.
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Backporting guide skipped a step about backport-open
label between backport-requested
and backported-to
.