edx sga: get_storage_class function was removed in Django 5.1

[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

https://github.com/awais786/edx-sga/blob/d64df6c949f8b3d7ec4ef9c2565723f61c6e3143/edx_sga/utils.py#L12

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 )

[awais786]

@feanil mitodl/edx-sga#368 please review.

[feanil]

Looks like this was released in mitodl/edx-sga#372 in 0.26.0 which is already live in edx-platform so I think this is good to go. Should there be a note added to the Ulmo release notes about the new sga_storage setting?