Update Admin Navigation Copy from 'Custom models' to 'Self-hosted models'

What does this MR do and why?

Updates Admin Navigation Copy from 'Custom models' to 'Self-hosted models'. This aligns better with current nonclemature used in other parts of the UI. i.e:

AI-powered features
--- GitLab Duo
--- Self-hosted models
--- Features

This is a follow up to #466605 (closed) for addressing this comment.

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