[9.20] chg: ci: Various autorebase improvements

  - Rewrite cherry-pick references during autorebases
  - Fix autorebase error reporting
  - Limit post-push pipelines for autorebased branches
  - Only autorebase when there is anything to rebase
  - Conflate missing commit reference notifications
  - Support autorebasing backported security MRs

Backport of MR !12024

Merge branch 'backport-michal/autorebase-improvements-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!12069