Remove the FF zoekt_cross_namespace_search

What does this MR do and why?

This change removes a feature flag called zoekt_cross_namespace_search that was controlling whether users could search for code across all projects globally (rather than just within specific projects or groups).

The feature flag was originally introduced to gradually roll out this global search capability, but now that the feature is stable and ready for all users, the flag is being removed. This means global code search is now permanently enabled for everyone.

The changes update documentation to reflect that the feature flag was removed in GitLab version 18.7, clean up code that was checking whether the flag was enabled or disabled, and simplify the logic since the feature is now always available. Tests were also updated to remove scenarios that were testing the old flag-controlled behavior.

In simple terms, a temporary control switch for a global code search feature has been removed because the feature is now ready to be permanently available to all users.

Customers can still turn off the global code search feature by the admin setting checkbox by unchecking the global code search checkbox. This is already unchecked for gitlab.com

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