We're transitioning from the boolean `namespace_details.pending_delete`
column to the new `deleted_at` timestamp column. This change improves
clarity, as `pending_delete` incorrectly suggests future scheduling
rather than an ongoing process.

The `deleted_at` column will now indicate when the background deletion
process for a group has started. This will also enable us to see when
the group was deleted to help in debugging if the group deletion fails.

Changelog: other