Disable signups for review apps

Disables signups in the review app once its been installed.

This is pulled from the work done here in the gitlab review apps: https://gitlab.com/gitlab-org/gitlab/-/blob/99ba0b38f504342ce8654cc1378b4e199cacef01/scripts/review_apps/review-apps.sh#L137

And is mostly kept the same, with the exception of the removal of the jq check at the end, as we don't have jq available in our current ci image here.

Closes https://gitlab.com/gitlab-org/charts/gitlab/-/issues/2272