Switch grading data format in CourseEnrollmentSerializer

[kaizoku]

This updates the format of the grading key from the CourseEnrollmentSerializer to remove the unnecessary wrapping in a list since it will always be a single item.

JIRA tickets: BB-3440

Merge deadline: Promptly

Testing instructions:
Check the format of the grading key under the course enrollments API and ensure it's not a list: http://localhost:18000/api/enrollment/v1/enrollment?user=honor

Author notes and concerns:

1. The final format of upstream changes this endpoint is still under discussion on https://github.com/edx/edx-platform/pull/20948/files

Reviewers

• 0x29a

[0x29a]

👍

• I didn't manage to test this because I have no ironwood devstack and setting up working fork locally will take too much time for such trivial change.
• I read through the code
• I checked for accessibility issues
• Includes documentation

[kaizoku]

Thanks @0x29a. I switched this to our common Ironwood branch since I found this customization is also a part of it.