Fix red develop 8caa73985a2e

The merge lane found `develop` red at `8caa73985a2e8ae4fd51304d4fac7475d6652c44` on its own: no queued PR merges until it is fixed. Failing (tests and gate stages): - `tests/test_tenant_offboard.py::test_every_org_scoped_table_is_in_the_purge_census` - `api-unit-dc-test` The whole gate log: artifact `12441` on lane task 11444. Make `develop` pass its full gate again. Open a PR as usual: the merge lane merges a PR that fixes some of these and adds no failure even while `develop` is red.

Purge census test now excuses _RETIRING_TABLES (email_mailboxes, left by Task 11433); offboard tests 7/7, quick gate green; PR #7395 queued to merge lane. Memory: none applied.