use state_variables_ordered API

it should be used over `(state_)variables` as it includes inherited, private variables, too.
fixes #1322

slither/tools/read_storage/read_storage.py

@@ -253,7 +253,7 @@ def get_all_storage_variables(self, func: Callable = None) -> None:
                     func,
                     [
                         (contract, var)
-                        for var in contract.variables
+                        for var in contract.state_variables_ordered
                         if not var.is_constant and not var.is_immutable
                     ],
                 )