Define BoundedContext for RemoteMirrors

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See documentation: https://docs.gitlab.com/ee/development/software_design.html#how-to-define-bounded-contexts

New set of RemoteMirrors services was implemented before BoundedContext restriction was defined.

We need to fix Rubocop warnings for them.

 Gitlab/BoundedContexts: Module RemoteMirrors is not a valid bounded context. See https://docs.gitlab.com/ee/development/software_design#bounded-contexts.