Implement cron job with new Task API

[jcharaoui]

The new Task API was introduced in recent Moodle versions to improve cron tasks. The legacy cron tasks will become deprecated in Moodle 3.5, and removed in Moodle 3.9.

It should be relatively easy to implement the grade import cron task into this API, and would allow platform administrators do reschedule it as needed, or disable the task altogether if required.