Cleanup backgroundUserNamespace migration

What does this MR do and why?

MR !71510 (merged) was merged into %14.5. This background migration is expected to take roughly 17 days much shorter due to auto-tuning.

So in %14.6, we need to have a cleanup migration in order to ensure it's finished, before we start removing the UserNamespace transition code in #341070 (closed)

We also add a change_column_null :namespaces, :type, false to add a tighter constraint.

Migration

up

> RAILS_ENV=test rake db:migrate:up VERSION=20211101222614
== 20211101222614 ConsumeRemainingUserNamespaceJobs: migrating ================
-- change_column_null(:namespaces, :type, false)
   -> 0.0011s
== 20211101222614 ConsumeRemainingUserNamespaceJobs: migrated (0.0311s) =======

down

> RAILS_ENV=test rake db:migrate:down VERSION=20211101222614
== 20211101222614 ConsumeRemainingUserNamespaceJobs: reverting ================
-- change_column_null(:namespaces, :type, true)
   -> 0.0014s
== 20211101222614 ConsumeRemainingUserNamespaceJobs: reverted (0.0014s) =======

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #344596 (closed)