Force page_access_level private in saas

What does this MR do and why?

See: https://jihulab.com/gitlab-cn/gitlab/-/issues/1222#note_2246025

Force pages visibility to be private when disable public access
So that user did not belongs to project and anonymous user can not access to jh pages

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

1. install pages by

gdk install gitlab-pages

1. enable pages
2. enable access control
3. disable public access
4. create a page and run

 project.project_feature.pages_access_level

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.