Implement automating backports

### Context

Part of https://github.com/canonical/bundle-kubeflow/issues/967

### What needs to get done

Implement what is defined in spec https://github.com/canonical/bundle-kubeflow/issues/969

### Definition of Done

There is automation for backporting PRs in all Charmed kubeflow repositories

Start by reading the linked parent issue 967 and specification issue 969, since this issue delegates the implementation details there. Then inspect how backporting should be automated across all Charmed Kubeflow repositories; done means the automation is available in every such repository.

Written by the indexing model from the issue text.