Allow cop rule `Gitlab/NamespacedClass` to ignore `Gitlab` as namespace

What does this MR do and why?

This MR allows cop 👮 rule Gitlab/NamespacedClass to ignore Gitlab and ::Gitlab.

They are now considered pseudo top-level and flags any class definition without additional domain namespace definition.

New exclusions based on the rule changes were added via

bundle exec rake rubocop:todo:generate[Gitlab/NamespacedClass]

Please see individual commits for more details and please don't squash the commits.

Refs Only allow classes and modules directly under t... (#299181 - closed)

Screenshots or screen recordings

n/a

How to set up and validate locally

REVEAL_RUBOCOP_TODO=0 bundle exec rubocop --only Gitlab/NamespacedClass

MR acceptance checklist

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