feat: deprecate get_storage_class in core/storage.py

[dwong2708]

Description

Fixes: #36739

The main goal is to modify the get_storage_class function referenced here in order to deprecate it.
However, a few special changes have been made, as described below:

• In this file, get_storage_class was replaced by the generic resolve_storage function.
• A similar update was made here.

Why does the get_storage_class function still exist with a new implementation that uses import_string?
This change is related to the DjangoStorageReportStore class and its parent implementation. Specifically, here, a dynamic logic is used to define the storage backend.

Given the complexity of that logic, I recommend creating a separate ticket to refactor this part of the code.

Lastly, the resolve_storage_backend function was updated to support a legacy_sec_setting_keys parameter. This allows it to handle storage settings defined as nested dictionaries. For example:

settings.VERIFY_STUDENT = {
    "STORAGE_CLASS": "storage.path.class"
}

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