Update Vault instructions for cluster rebuild
- We now force our clusters to build the secrets for us, this is now hard coded
- Let's update our docs to account for this as the service account and secret are not linked, thus not queryable at the moment.
- Work to improve this in the future is here: https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/16931
Addresses: https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/16922