Replace allow_anonymous_searches ff with admin setting

What does this MR do and why?

• Replace allow_anonymous_searches ff with admin setting.
• Add a setting named Enable search for unauthenticated users in the UI
• Rename Global Search Pane to Visibility & access controls

References

Screenshots or screen recordings

Mode	Before	After
EE
CE

How to set up and validate locally

1. Update the setting from admin panel
2. Try to doing search as anonymous user and notice you get redirected to login page.

MR acceptance checklist

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

Related to #434218 (closed)