Validate the NOT NULL constraint on gitlab_subscriptions namespace_id

This is a followup issue of MR !54319 (merged), to Add not null constraint to gitlab_subscriptions namespace_id.

As per the suggested approach in document https://docs.gitlab.com/ee/development/database/not_null_constraints.html#add-a-not-null-constraint-to-an-existing-column, it requires two steps in two releases.

MR !54319 (merged) addressed the required actions for current release.

While this issue is about the second part in next release to Validate the NOT NULL constraint using a post-deployment migration