Changed the backport logic so that the bot only pushes the backport branch if it does not already exist in the remote fork.
If the branch exists, the bot skips the push, allowing only users to update (force-push) the branch after the backport PR is open.
Fixes: https://github.com/scylladb/scylladb/issues/24953
Closes scylladb/scylladb#24954
(cherry picked from commit ed7c7784e4)
Closes scylladb/scylladb#24967