Submission validation is checking for Target checklist instead of Assignment checklist

[Yaswanth Kumar Bethu]

Describe the bug
In phase-2 we had moved the target checklist to the Assignment checklist but did not update some methods in the submission validator to use the Assignment checklist.

To Reproduce
Steps to reproduce the behavior:

1. Go to target with Assignment and update the existing checklist, like deleting a present one. This will make a difference between the Target checklist and Assignment checklist.
2. Now try submitting the form for that target, it will show an error Missing answer for question:.. for deleted item in the checklist.
3. This is because of it presence in target checklist and submission validation checking for target instead of Assignment

   pupilfirst/app/graphql/concerns/validate_student_submission.rb

   Line 44 in fd0222a

    target 

Expected behavior
It should check for assignment checklist instead of the target checklist.

Screenshots
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