Here is the plan: * [x] Migrate the code of the mill-plugin to a separate repository, see also #2704 * Done: https://github.com/scala-steward-org/mill-plugin * [x] Decide an a version scheme for external mill-plugin and make a first release * Done: first release version: `0.17.0` * [x] Refactor scala-steward code to use the new external plugin * PR: https://github.com/scala-steward-org/scala-steward/pull/2821 * PR: https://github.com/scala-steward-org/scala-steward/pull/2823 * [x] Remove mill-plugin module from this project * [x] ~Update documentation~ (there is none affected)