Draft: Prevent instantiation of Namespace objects

What does this MR do and why?

An experiment in preventing instantiation of Namespace objects.

This works for me in console:

> Namespace.new
=> #<Namespaces::UserNamespace:0x000055f82cd40588
 id: nil,
 name: nil,
 path: nil,
 owner_id: nil,
 created_at: nil,
 updated_at: nil,
 type: "User",
#... <snip>

it also works for becomes:

> Namespaces::ProjectNamespace.new.becomes(Namespace)
RuntimeError: Cannot directly instantiate a Namespace
from /home/charlie/Projects/gitlab-development-kit/gitlab/app/models/namespace.rb:9:in `initialize'

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How to set up and validate locally

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

MR acceptance checklist

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Related to #341070 (closed)