[Backport 2023.1] feature(auto-backport.py): replace Mergify

[mergify]

Adding an auto-backport.py script to handle backport automation instead of Mergify.

The rules of backport are as follows:

• Merged PR 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
• All PRs will be open under the scylladbbot account - which will allow write access in case of conflicts
• PR with conflicts will open in draft mode.
• Fixing cherry-pick the wrong commit SHA. With the new script, we always take the SHA from the stable branch

Since the SCT repo is public and GitHub doesn't clone permissions to the public repo (for forks),
For the first time only, each PR owner will receive an email
with an invite to the scylladbbot/scylla-cluster-tests fork

(cherry picked from commit 2f132a8 )

Parent PR: #9697

[mergify]

Cherry-pick of 2f132a8 has failed:

On branch mergify/bp/branch-2023.1/pr-9697
Your branch is up to date with 'origin/branch-2023.1'.

You are currently cherry-picking commit 2f132a86.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	new file:   .github/scripts/auto-backport.py

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   .github/scripts/search_commits.py
	deleted by us:   .github/workflows/add-label-when-promoted.yaml

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally