Follow-up from "Correct inconsistency between namespace_id and project_namespace_id on design management tables"

The following discussion from !202696 (merged) should be addressed:

• @mdangelo6 started a discussion: (+9 comments)

  Hi @mcelicalderonG 👋 I noticed this spec failing in this MR, which is attempting to fix the sharding key in the design_management tables.

  In summary, we're trying to change the sharding key on design_management_designs from projects.namespace_id to projects.project_namespace_id as part of #557010 (closed). I noticed that the notifications table here fetches its sharding key from the design_management_designs table. I'm wondering if this means we'll need to requeue the background migration for the p_sent_notifications table once this BBM completes?