WIP: Clarify that USE_STATIC_IP is mandatory for letsencrypt usage

What does this MR do?

Clarify that USE_STATIC_IP=true is mandatory if the user wants to use Lets Encrypt. When that parameter is not passed, and user proceeds to installing GitLab, the pods for obtaining the certificate fail, the LoadBalancer remains in a Pending state, and it's hard to debug and fix that after the cluster has been created.

Related issues

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened