Ensure description generation is in alignment with our AI unit primitives

Problem

We need to ensure the license/permission checks for description generation are in line with our unit primitives (https://gitlab.com/groups/gitlab-org/-/epics/12556+)

Proposal

• Update access_data.yml, move description_generation to separate section
• Update cloud_connector.yml to match
• Update ee/app/policies/ee/global_policy.rb