CORS error on `shopify app dev` on the cli reverse proxy with `credentials: "include"`

Hello, following this change, our local setup no longer works because we use credentials: "include" on some of our fetch requests to our backend.

This PR removes our Access-Control-Allow-Credentials: true header

Originally posted by @Dylan-GJ in https://github.com/Shopify/cli/issues/7164#issuecomment-5207778050