Point GitLab Rails app to cloud.gitlab.com/ai

We decided to let all traffic to the AI gateway go through a load-balanced cloud.gitlab.com/ai endpoint instead of going directly to codesuggestions.gitlab.com, as documented here. The work for standing up the LB is defined in https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/24711.

We need to:

• Update all URLs in gitlab-rails from codesuggestions.gitlab.com to cloud.gitlab.com/ai
• Update any env vars that currently set CODE_SUGGESTIONS_BASE_URL directly if necessary
• Update relevant documentation and mention in the release post that web proxies etc. need to reflect this change