Adding AWS note to 15.0
What does this MR do and why?
Adding the note from this page → https://docs.gitlab.com/omnibus/update/gitlab_15_changes.html#aes256-gcm-sha384-ssl-cipher-no-longer-allowed-by-default-by-nginx as I recently had an emergency where this was missed in their upgrade plans to 15.0.5. I feel having this here will help others that are planning their upgrade as this is an important step if using an AWS Load Balancer.