docs: add section on json and defer being deprecated

docs/upgrading/v6.md

@@ -293,6 +293,18 @@ function shouldRevalidate({ actionStatus, defaultShouldRevalidate }) {
 }
 ```
 
+## Deprecations
+
+The `json` and `defer` methods are deprecated in favor of returning raw objects.
+
+```diff
+async function loader() {
+- return json({ data });
++ return { data };
+```
+
+If you were using `json` to serialize your data to JSON, you can use the native [Response.json()][response-json] method instead.
+
 ## Upgrade to v7
 
 Now that your app is caught up, you can simply update to v7 (theoretically!) without issue.
@@ -354,3 +366,5 @@ Note you should use a top-level import for non-DOM contexts, such as Jest tests:
 Congratulations, you're now on v7!
 
 [react-flushsync]: https://react.dev/reference/react-dom/flushSync
+[response-json]: https://developer.mozilla.org/en-US/docs/Web/API/Response/json
+[data-util]: https://api.reactrouter.com/v7/functions/react_router.data.html