Hey there! The Edit DNS Zone interface was removed in cPanel v90 and replaced with the DNS Zone Manager interface provided in your screenshot. More on this can be found here:
How to edit a DNS record in the DNS Zone Manager
How to edit a DNS record in the DNS Zone Manager
Yes, we've published an article with a guide on configuring custom nameservers here:
How to configure nameservers in a cPanel & WHM server
How to configure nameservers in a cPanel & WHM server