Use GFKSerializerField for ObjectChangeSerializer.changed_object and EventRuleSerializer.action_object

contrib/openapi.json

@@ -219028,9 +219028,8 @@
                         "nullable": true
                     },
                     "action_object": {
-                        "type": "object",
-                        "additionalProperties": {},
-                        "readOnly": true
+                        "readOnly": true,
+                        "nullable": true
                     },
                     "description": {
                         "type": "string",
@@ -229795,7 +229794,11 @@
                         "format": "int64"
                     },
                     "changed_object": {
-                        "nullable": true,
+                        "readOnly": true,
+                        "nullable": true
+                    },
+                    "object_repr": {
+                        "type": "string",
                         "readOnly": true
                     },
                     "message": {
@@ -229820,6 +229823,7 @@
                     "display_url",
                     "id",
                     "message",
+                    "object_repr",
                     "postchange_data",
                     "prechange_data",
                     "request_id",