Fixed #25852 -- Made sure AlterModelManager forces a reload of its model state.

Thanks to Geoffrey Sechter and the Django NYC group for the report and Markus
for the review.

django/db/migrations/operations/models.py

@@ -590,6 +590,7 @@ class AlterModelManagers(Operation):
     def state_forwards(self, app_label, state):
         model_state = state.models[app_label, self.name_lower]
         model_state.managers = list(self.managers)
+        state.reload_model(app_label, self.name_lower)
 
     def database_forwards(self, app_label, schema_editor, from_state, to_state):
         pass