Correct the type of FileField.storage (#731)

* Correct the type of FileField.storage

This instance property can't be a callable. Although the FileField constructor
allows a callable, it is immediately resolved to an instance of Storage.
See: f5802a21c4/django/db/models/fields/files.py (L231-L235)

* Correct the type of FieldFile.storage

This instance property is copied directly from `FileField.storage` and should be
the same type.
See: f5802a21c4/django/db/models/fields/files.py (L21)

django-stubs/db/models/fields/files.pyi

@@ -3,14 +3,14 @@ from typing import Any, Callable, Iterable, Optional, Type, TypeVar, Union, over
 
 from django.core.files.base import File
 from django.core.files.images import ImageFile
-from django.core.files.storage import FileSystemStorage, Storage
+from django.core.files.storage import Storage
 from django.db.models.base import Model
 from django.db.models.fields import Field, _ErrorMessagesToOverride, _FieldChoices, _ValidatorCallable
 
 class FieldFile(File):
     instance: Model = ...
     field: FileField = ...
-    storage: FileSystemStorage = ...
+    storage: Storage = ...
     def __init__(self, instance: Model, field: FileField, name: Optional[str]) -> None: ...
     file: Any = ...
     @property
@@ -33,7 +33,7 @@ class FileDescriptor:
 _T = TypeVar("_T", bound="Field")
 
 class FileField(Field):
-    storage: Union[Storage, Callable[[], Storage]] = ...
+    storage: Storage = ...
     upload_to: Union[str, Callable] = ...
     def __init__(
         self,