docs: add cookies section in v5 migration (nextauthjs#11722)

* Add cookie documentation migrating-to-v5.mdx

Some projects might use the cookies in a middleware. It is important to mention the refactor.

* formatting

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Co-authored-by: Thang Vu <hi@thvu.dev>

docs/pages/getting-started/migrating-to-v5.mdx

@@ -345,6 +345,10 @@ export type {
 
 </Steps>
 
+## Cookies
+
+- The `next-auth` prefix is renamed to `authjs`.
+
 ## Summary
 
 We hope this migration goes smoothly for everyone! If you have any questions or get stuck anywhere, feel free to create [a new issue](https://github.com/nextauthjs/next-auth/issues/new) on GitHub, or come chat with us in the [Discord](https://discord.authjs.dev) server.