RemovedInDjango51Warning for get_storage_class in COURSE_IMPORT_EXPORT_STORAGE.

[awais786]

The get_storage_class function was removed in Django 5.1:

https://docs.djangoproject.com/en/5.2/releases/5.1/#features-removed-in-5-1

The file cms/djangoapps/contentstore/storage.py still uses get_storage_class, which is now deprecated and needs to be replaced.

Example fix in edx-platform PR #36628
Reference implementation in edx-val commit

Notes:

• Ensure the updated code is compatible with both Django 4.2 and Django 5.2.
• Add or update relevant unit tests to cover the changes.
• Verify functionality locally before merging ( course import export from studio )

This subticket export_course_metadata might be interlinked.

@feanil Please review the ticket description and let me know if any modifications or additions are needed.