If WHMCS tells you to upgrade your server's version of cURL to prevent security issues, do not be alarmed.
cURL 7.35.0 is the most recent and secure version for Ubuntu 14.04.
cURL 7.47.0 is the most recent and secure version for Ubuntu 16.04.
cURL 7.58.0 is the most recent and secure version for Ubuntu 18.04.
Like most Linux distributions, Ubuntu backports security patches so that software like cURL can stay secure without breaking other software by changing the version.
You can find the latest version number of cURL for your Ubuntu version below.