fix: remove expired license status and define EnrollmentDueDateSerializer

[adamstankiewicz]

Description:

• Defines EnrollmentDueDate serializer to reflect the specific fields it'd return, if there's a non-empty list (technically a no-op, since the enterprise_course_enrollments API hardcoded an empty list in its response). Opted to be explicit to match the newly defined TypeScript interface on the consuming Learner Portal.
• Removes expired subscription license status from the BFF serializer/responses since it's not actually a valid license status.

Merge checklist:

• ./manage.py makemigrations has been run
  • Note: This must be run if you modified any models.
    • It may or may not make a migration depending on exactly what you modified, but it should still be run.

Post merge:

• Ensure that your changes went out to the stage instance
• Deploy to prod instance