For GA release: Redefine ai_catalog feature as a GA AI Feature, update UI labels, feature flags and docs

About

As part of https://gitlab.com/gitlab-org/gitlab/-/issues/568176 we added ai_catalog to a list of BETA AI features, so the AI Catalog would only be available to projects that had opted-in to BETA AI features.

In order to release to GA we need to switch it over to be listed as GA.

Steps

1. In !215502 (merged) we are
2. We still need to ship ai_catalog_flows as enabled (will happen in !214872 (merged))
3. There will be some clean-up work which can happen when we come to remove the ai_duo_agent_platform_ga_rollout flag, which can be a follow-up
   • Update Gitlab:Llm::Utils::AiFeaturesCatalogue::LIST to define ai_catalog as maturity: :ga. This will mean customers who have not opted into experimental/beta features can begin to use our features
   • Remove all experimental/beta checks from Ai::Catalog.available? added in !196066 (merged)
     • Search the codebase for https://gitlab.com/gitlab-org/gitlab/-/issues/570161 to find TODOs for code to remove
   • We may be able to change scope of the ai_catalog_available condition in GlobalPolicy - see !196066 (comment 2906558543)

Proposal

• Have AI Catalog support ai_duo_agent_platform_ga_rollout being enabled to switch it to GA
• Remove experiment labels.
• Update documentation to mention GA status.
• Set all feature flags of features that are going GA to default enabled (or remove them, but might be good to keep the around for a milestone in case we or our customers need to toggle something off)