Adding an auto-backport.py script to handle backport automation instead of Mergify.
The rules of backport are as follows:
* Merged or Closed PRs with any backport/x.y label (one or more) and promoted-to-master label
* Backport PR will be automatically assigned to the original PR author
* In case of conflicts the backport PR will be open in the original autoor fork in draft mode. This will give the PR owner the option to resolve conflicts and push those changes to the PR branch (Today in Scylla when we have conflicts, the developers are forced to open another PR and manually close the backport PR opened by Mergify)
* Fixing cherry-pick the wrong commit SHA. With the new script, we always take the SHA from the stable branch
* Support backport for enterprise releases (from Enterprise branch)
Fixes: https://github.com/scylladb/scylladb/issues/18973
(cherry picked from commit f9e171c7af)
Closesscylladb/scylladb#21470
160823ccaf