2.73.0

Added

• Formal release channels. Every release is now git-tagged and published
  as a GitHub pre-release with its changelog section as notes
  (scripts/publish_release.sh). Soaked releases are promoted to stable
  (scripts/promote_release.sh), which moves the new :stable and
  :stable-cpu Docker tags without rebuilding. docs/releasing.md
  documents the flow and the Breaking changelog convention (issue #567
  groundwork).