Change default Namespace type from nil to User

What does this MR do and why?

Change default for type column for namespaces Namespaces in DB. Change default to 'User'.

DB migrate output

up

$ bundle exec rails db:migrate:up VERSION=20211116091751
== 20211116091751 ChangeNamespaceTypeDefaultToUser: migrating =================
-- transaction_open?()
   -> 0.0000s
-- change_column_default(:namespaces, :type, "User")
   -> 0.0089s
== 20211116091751 ChangeNamespaceTypeDefaultToUser: migrated (0.0246s) ========

down

$ bundle exec rails db:migrate:down VERSION=20211116091751
== 20211116091751 ChangeNamespaceTypeDefaultToUser: reverting =================
-- transaction_open?()
   -> 0.0000s
-- change_column_default(:namespaces, :type, nil)
   -> 0.0161s
== 20211116091751 ChangeNamespaceTypeDefaultToUser: reverted (0.0315s) ========

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.

BLOCKED BY !74152 (merged) and !73495 (merged)

(No longer blocked as we've changed what this MR does - it only sets the default namespace type to User instead of directly migrating any data)

Related to #345105 (closed)