Set upstream review app test to be manual

We've had a lot of failed review app pipelines in gitlab due to the upstream nav test job, and @kpaizee figured out that it's due to the MR's branch being slightly out of sync with the gitlab-docs main branch. So if someone adds/updates a global nav entry update, but someone triggers a review app from a slightly older feature branch in gitlab, the global nav pipeline will fail.

As a quick MVC, let's set this job to be manual, so that we can still use it when cleaning up redirects as part of chores, without failing for users that haven't actually deleted any docs.