`project_compliance_standards_adherence.namespace_id` FK preventing group deletion

Introduced in !122293 (merged), project_compliance_standards_adherence.namespaces_id column is NOT NULL but the foreign key is set to ON DELETE SET NULL which prevents the deletion of some groups (Sentry link).

Error

PG::NotNullViolation: ERROR:  null value in column "namespace_id" of relation "project_compliance_standards_adherence" violates not-null constraint (ActiveRecord::NotNullViolation)

Suggestion

The current code is sufficient. Bug is happening due to misaligned namespace_id fields due to incident. => Fix rows with background migration.

Details in related MR: !176704 (comment 2284416186)