Set variable to new domain for docs review apps (GitLab)

What does this MR do and why?

In gitlab-docs!3179 (merged), we set the docs project to start deploying review apps to a GCP bucket, but it wasn't working because we also forgot to update this CI/CD variable to the new location.

This MR updates the variable, and should be merged around the same time as gitlab-docs!3198 (merged), which re-implements the new review app system.

• Related to gitlab-docs!3179 (merged)
• Related to gitlab-docs#735 (closed)

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