Fix GitLab Migration groups & projects visibility levels to be preserved

What does this MR do and why?

This MR fixes an issue with GitLab Migration (aka Bulk Imports) visibility settings inconsistency where migrated groups & projects lose their visibility settings and typically get 'private' visibility setting assigned, regardless of the original value. Changes in this MR preserve visibility settings, while taking into consideration group & instance visibility limitations (e.g. if a public group is imported into an internal group it's visibility is lowered to internal).

Mentions #331252 (closed)

Screenshots or screen recordings

Public into public group	public_group_v2.mov
Public into internal group	internal_group.mov
Public into private group	private_group.mov

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.

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