<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Work on this issue](https://contributors.gitlab.com/manage-issue?action=work&projectId=278964&issueIid=534710) - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=534710) </details> <!--IssueSummary end--> Issue to implement data migration related to https://gitlab.com/gitlab-org/gitlab/-/issues/525342 Data migration: * Delete `Import::SourceUser` records where the `namespace` is a personal namespace * Delete all placeholder references (`Import::Placeholders::Membership` and `Import::SourceUserPlaceholderReference`) that are associated with those `Import::SourceUser` records.