Replace str_contains() call on moodle branch with PHP 7.4-compatible equivalent

[Limekiller]

Recently a change was made to the library included in the Moodle plugin mod_hvp that introduces a function call to str_contains(), which is only compatible with PHP 8.0 and higher. However, the plugin claims to support Moodle versions that do not support PHP 8.0, as well as Moodle 4.1, which is still actively supported by Moodle and is fully compatible with PHP 7.4.

Moodle sites on the actively-supported Moodle 4.1 that are running on PHP 7.4, or older sites that do not support PHP 8.0 but are claimed to be supported by mod_hvp, will encounter errors until the function call is removed. I have created a pull request to do so here.