In the github mediawiki-vendor- master is now a less 1.80 .
But at github is https://github.com/wikimedia/less.php/archive/master.zip (1.8.1 with less 2.53), which is php 7.3 ready
require_once "$IP/vendor/wikimedia/less.php/lib/Less/Autoloader.php";
Less_Autoloader::register();