Fix Ai::Catalog.available? GA check for self-managed instances

What does this MR do and why?

This MR fixes the fallback logic in Ai::Catalog.available? for Self-Managed/Dedicated instances to fallback to ::Gitlab::CurrentSettings.instance_level_ai_beta_features_enabled? rather than a hard-coded value. This tweak helps ensure AI Catalog doesn't ship as GA in 18.7.

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Related to #570161