IP addresses to block to stop WP auto-update?

You don’t need to block the IP address at all.

Append this code in wp-config.php of your website in case you have access to it::

define('WP_AUTO_UPDATE_CORE', false);

Alternative Method
This will hide update messages for all kind of users::

Add this function to your functions.php

function hide_update_notice_to_all_but_admin_users()
{
    if (current_user_can('read')) {
        remove_action( 'admin_notices', 'update_nag', 3 );
    }
}
add_action('admin_head', 'hide_update_notice_to_all_but_admin_users', 1);