Make Duo Code Review available as Beta (AI catalogue, docs)

1. Mark the feature as beta in the AI features catalogue: https://gitlab.com/gitlab-org/gitlab/-/blob/e3b7f13ac2dffb09961f731098391f055d3d4f5f/ee/lib/gitlab/llm/utils/ai_features_catalogue.rb#L115
2. Update docs to reflect that it's now Beta