Move cube_api_proxy feature flag to EE

What does this MR do and why?

Related to #381358 (closed)

The cube_api_proxy feature flag is EE only, it was created originally on CE.

Screenshots or screen recordings

No visual changes.

How to set up and validate locally

1. Make sure you are on GitLab Ultimate.
2. Enable the product_analytics_internal_preview feature flag

   Feature.enable(:product_analytics_internal_preview)

3. Visit any PA project pages such as http://127.0.0.1:3000/flightjs/Flight/-/product_analytics/dashboards/ (the page uses the cube_api_proxy feature flag)
4. The dashboards page should be presented.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.