Validate NOT NULL constraint on gitlab_subscriptions namespace_id

What does this MR do?

For issue #321662 (closed). This MR is the seconds step to validate the NOT NULL constraint on GitlabSubscriptions namespace_id column.

Isseu #243496 (closed) requires to add NOT NULL constraint on GitlabSubscriptions namespace_id column. Per 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.

The first step is done by MR !54319 (merged), where it:

• Ensure the constraint is enforced at the application level (i.e. add a model validation).
• Add a post-deployment migration to add the NOT NULL constraint with validate: false.
• Add a post-deployment migration to fix the existing records

This MR is the second step, where it:

• Validate the NOT NULL constraint using a post-deployment migration.

Migration output

Up

$ bin/rails db:migrate
== 20210322115438 ValidateNotNullConstraintOnGitlabSubscriptionsNamespaceId: migrating 
-- current_schema()
   -> 0.0002s
-- execute("ALTER TABLE gitlab_subscriptions VALIDATE CONSTRAINT check_77fea3f0e7;")
   -> 0.0006s
== 20210322115438 ValidateNotNullConstraintOnGitlabSubscriptionsNamespaceId: migrated (0.0029s) 

Down (Note: rollback is NO-OP in this migration script)

$ bin/rails db:rollback
== 20210322115438 ValidateNotNullConstraintOnGitlabSubscriptionsNamespaceId: reverting 
== 20210322115438 ValidateNotNullConstraintOnGitlabSubscriptionsNamespaceId: reverted (0.0000s) 

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

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Related to #321662 (closed)