Function libxml_disable_entity_loader() is deprecated in php 8.0

it's used quite a few times in moodle

According to https://php.watch/versions/8.0/libxml_disable_entity_loader-deprecation it is no longer needed in PHP 8.0 however still needed in previous versions, they recommend to wrap it like this:

- libxml_disable_entity_loader(true);

+ if (\PHP_VERSION_ID < 80000) {

+      libxml_disable_entity_loader(true);

+ }

Maybe we need to create a single wrapper function and remove all usages of it when minimum PHP version is raised to 8?