Column organization_users.organization_id should be ready for Cells 1.0

Column organization_users.organization_id should meet all of the following requirements for Cell 1.0:

• has no default
• is not nullable
• has a foreign key to organizations

Once the above is done we can remove the table from https://gitlab.com/gitlab-org/gitlab/-/blob/master/spec/lib/gitlab/database/sharding_key_spec.rb#L182-199.