CiBot's PHP Linter needs to run different versions of PHP

It seems that CiBot's PHP Linter throws an error when running tests on code for Moodle 4.1. The cause seems to be that Moodle 4.1 requires PHP 7.4, but Linter seems to be running utilising PHP 7.3 and results in an error.

This issue was detected when performing a library up grade for Markdown MDL-75470. The library upgrade has introduced type annotations and CiBot was detecting an error.