Update config for running local AI Gateway

What does this MR do and why?

Update config for running local AI Gateway

• Previously, the recommendation was to set the URL in the database
• The database attribute is really meant for customers using self-hosted models. Most of the time, GitLab developers want to mimic the experience of GitLab customers using the "hosted" AI Gateway.
• The CLOUD_CONNECTOR_BASE_URL only works if the AIGW instance is available at the /ai path, which is specific to Cloud Connector.
• So now we are recommending a new ENV var for configuring this value.

References

• I am also updating AIGW setup instructions in GDK at gitlab-development-kit!5346
• see conversation in !204287 (closed)

How to set up and validate locally

1. Set the ENV var DEVELOPMENT_AI_GATEWAY_URL to your local AIGW
2. Make sure AI_GATEWAY_URL is unset and Ai::Setting.instance.ai_gateway_url is nil
3. Restart AI Gateway
4. Classic Duo Chat should still work

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