Merge branch 'backfill-desired-sharding-key-small-table-design_management_designs' into 'master'

Add and backfill namespace_id for design_management_designs

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/153149



Merged-by: Manoj M J <mmj@gitlab.com>
Approved-by: Manoj M J <mmj@gitlab.com>
Approved-by: Pavel Shutsin <pshutsin@gitlab.com>
Co-authored-by: Abhilash Kotte <akotte@gitlab.com>