fix: Check preferred_course_run_key for normalized_metadata

[brobro10000]

Description:
Takes into account the preferred_course_run_key when extracting the normalized_metadata from assignments.

Jira:
ENT-9688

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