update start_certificate_generation to DRF

[awais786]

Test to verify via postman

1. Try this URL in postman

expected result

{
    "message": "Certificate generation task for all students of this course has been started. You can view the status of the generation task in the \"Pending Tasks\" section.",
    "task_id": "449df233-3e34-48e6-8d11-64f1a334fb57"
}

---

verify by instructor dashboard.