Rename CODE_SUGGESTIONS_BASE_URL to AI_GATEWAY_URL
Problem
In GitLab-Rails, we allow to override AI Gateway URL by CODE_SUGGESTIONS_BASE_URL
environment variable. But AI Gateway is used by Duo Chat as well and it's misleading.
Proposal
Rename CODE_SUGGESTIONS_BASE_URL
to AI_GATEWAY_URL
. Make sure it doesn't result in a breaking change.