Today we send a reminder to PR's author when backport PRs has conflicts. Often, PR authors wait for their PR to be reviewed/merged, but the merge is not happening because the PR now conflicts with master and so maintainers won't merge it. This can lead to a stall, where maintainers wait for the author to rebase and authors are waiting for merge. In this PR we added the ability to notify the PR author as soon as base branch moved forward and rebase is requried Fixes: https://github.com/scylladb/scylla-pkg/issues/4955 Closes scylladb/scylladb#24209