Rename "Restrict the AI Catalog to a group hierarchy" to "Confine..." to avoid ambiguity with Restricted Visibility feature

What does this MR do and why?

This MR renames the setting we've described as "Restrict the AI Catalog to a group hierarchy" to be "Confine..." to avoid ambiguity with the new Restricted Visibility feature, which is unrelated.

Screenshots or screen recordings

Before	After

How to set up and validate locally

At a top-level group, go Settings > GitLab Duo.

Select Change configuration button.

Scroll down and notice the setting label has changed.

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