docs: switch to http.request.cookies for gradual rollout

[hochoy]

Summary

From this issue: #25478
Documentation of fields: https://developers.cloudflare.com/ruleset-engine/rules-language/fields/reference/http.request.cookies/

This pull request updates documentation examples to use the correct syntax for accessing cookie values in Cloudflare Workers. The changes clarify how to retrieve the first value of a cookie from the request object.

Documentation corrections:

• Updated the example values for setting the Cloudflare-Workers-Version-Key header to use http.request.cookies["session_id"][0] and http.request.cookies["user_id"][0] instead of the incorrect http.cookie syntax in src/content/docs/workers/static-assets/routing/advanced/gradual-rollouts.mdx. [1] [2]

Screenshots (optional)

Before

After

Logs of CF worker

Documentation checklist

• The documentation style guide has been adhered to.

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