Add callout "transition_to_jihu_callout" on Backend

What does this MR do and why?

Related Issue: https://gitlab.com/gitlab-jh/gitlab-jh-enablement/-/issues/192

Related JiHu Issue: https://jihulab.com/gitlab-cn/gitlab/-/issues/3930

Related Front-end MR: !142126 (merged)

Add a callout to control a pop-up banner of Jihu transition.

This banner only appears on the /admin and /admin/projects pages and only displays after a series of other conditions are met.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

No UI changes in this MR.

Final effect (after merging the front-end code):

How to set up and validate locally

1. Make sure your local environment meets these conditions
2. Visit the /admin and /admin/projects pages, open the browser console, and expect to see the .js-jh-transition-banner element

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cc @chaomao @xfyuan @JeremyWuuuuu