I’m brand new to nginx and just ran into this problem myself and spent some hours trying to figure it out. My solution feels hacky and theres probably a better way to do this, but it got it working on my server.

In the first ‘server’ block, right below the server names I added a conditional for a redirect. Here’s what it would look like using your code.

server {
    root /var/www/mydomain.com/html;
    index index.html index.htm index.nginx-debian.html;

    server_name mydomain.com www.mydomain.com;


    if ($host = www.mydomain.com) {
        return 301 https://mydomain.com$request_uri;
    }

    #Certbot stuff here...
}

If someone knows of or finds a better way to do this, please leave a comment.