fix: drop superadmin entries for user assigments endpoint

[dcoa]

Description

Follows the same approach as #345, which dropped superadmin entries from the assignment list endpoint (/api/authz/v1/assignments/). This applies it to the single-user endpoint, /api/authz/v1/users/<username>/assignments/.

TeamMemberAssignmentsAPIView.get no longer callsget_superadmin_assignments(user_external_keys=[username]). It now returns role assignments only, from get_visible_user_role_assignments_filtered_by_current_user.

Related information

Fixes openedx/frontend-app-admin-console#180

Testing Instructions

• Log in as a staff/superuser or regular user.
• Call GET /api/authz/v1/users/<superadmin_username>/assignments/ and verify no django.staff or django.superuser entries appear.
• If you enter the admin console with a regular user with manage team permissions you should be able to remove the superadmin user for the selected scope.

Merge checklist:
Check off if complete or not applicable:

• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Fixup commits are squashed away
• Unit tests added/updated
• Manual testing instructions provided
• Noted any: Concerns, dependencies, migration issues, deadlines, tickets

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[mariajgrimaldi]

LGTM! When you can, could you bump the version? A minor patch would work since this is unused data. Thanks!

[dcoa]

thanks @mariajgrimaldi for reviewing the PR, I had bumped the version.