Backfill ProjectNamespace for every Project

We need a background migration to create a ProjectNamespace for every project based in the architecture blueprint https://docs.gitlab.com/ee/architecture/blueprints/consolidating_groups_and_projects/

• Backfill migration should set the current date for backfilled project namespaces created/updated date/times(i.e. not the project's created/updated date/times) re gitlab-org/manage/general-discussion#17406 (comment 700152960)