Fix resolving namespace conflicts on creating internal user

What does this MR do and why?

Related to #363813 (closed)

While investigating customer issue https://gitlab.com/gitlab-com/sec-sub-department/section-sec-request-for-help/-/issues/276, we found that the root cause of issue was unsuccessful Ghost User creation(Users::Internal.ghost), see https://gitlab.com/gitlab-com/sec-sub-department/section-sec-request-for-help/-/issues/276#note_1924606159. ) There reason why creation of the the Ghost User failed is that group with ghost namespace(Namespace.by_path('ghost')) was created prior to creation of the the Ghost User. While creating internal users the existing implementation does not check namespaces paths to resolve conflicts. When there is a a group created with namespace path as ghost, alert-bot, migration-bot, GitLab-Security-Bot, support-bot, automation-bot, GitLab-Llm-Bot, GitLab-Admin-Bot prior creating internal users for those usernames, it will raise the error. We found out that it has been known issue for a while, see #363813 (closed).

This MR fixes resolving namespace conflicts on creating internal user.

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