docs(migrating): fix broken react-router link (#43843)

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Fix broken link in migrating/from-react-router.md
- https://nextjs.org/docs/migrating/from-react-router#code-splitting
- https://nextjs.org/docs/migrating/from-react-router#scroll-restoration

The current link is shown as 404 because it is redirected to the
[document in react-router v6](https://reactrouter.com/en/main).

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see
[`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the
feature request has been accepted for implementation before opening a
PR.
- [ ] Related issues linked using `fixes #number`
- [ ]
[e2e](https://github.com/vercel/next.js/blob/canary/contributing/core/testing.md#writing-tests-for-nextjs)
tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see
[`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)

## Documentation / Examples

- [x] Make sure the linting passes by running `pnpm build && pnpm lint`
- [x] The "examples guidelines" are followed from [our contributing
doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

docs/migrating/from-react-router.md

@@ -134,7 +134,7 @@ Next.js has built-in support for [Server-side Rendering](/docs/basic-features/pa
 
 ## Code Splitting
 
-Next.js has built-in support for [Code Splitting](https://reactrouter.com/web/guides/code-splitting). This means you can remove any instances of:
+Next.js has built-in support for [Code Splitting](https://v5.reactrouter.com/web/guides/code-splitting). This means you can remove any instances of:
 
 - `@loadable/server`, `@loadable/babel-plugin`, and `@loadable/webpack-plugin`
 - Modifications to your `.babelrc` for `@loadable/babel-plugin`
@@ -145,7 +145,7 @@ For more information, read about [Dynamic Imports](https://nextjs.org/docs/advan
 
 ## Scroll Restoration
 
-Next.js has built-in support for [Scroll Restoration](https://reactrouter.com/web/guides/scroll-restoration). This means you can remove any custom `ScrollToTop` components you have defined.
+Next.js has built-in support for [Scroll Restoration](https://v5.reactrouter.com/web/guides/scroll-restoration). This means you can remove any custom `ScrollToTop` components you have defined.
 
 The default behavior of `next/link` and `next/router` is to scroll to the top of the page. You can also [disable this](https://nextjs.org/docs/api-reference/next/link#disable-scrolling-to-the-top-of-the-page) if you prefer.