Replace the legacy openai_experimentation FF with ai_global_switch

What does this MR do and why?

Replace the legacy openai_experimentation FF with ai_global_switch

This replaces the legacy openai_experimentation FF usages with the more generic ai_global_switch ops FF

Note that ai_global_switch FF was enbaled on production: https://gitlab.slack.com/archives/C101F3796/p1698324497375889

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