docs: change `NextAuth.js` to `Auth.js` (#72201)
## Summary
[`NextAuth.js`](https://next-auth.js.org/) is becoming
[`Auth.js`](https://authjs.dev/)!
Then update new documentation link.
### Improving Documentation
- [x] Run `pnpm prettier-fix` to fix formatting issues before opening
the PR.
- [x] Read the Docs Contribution Guide to ensure your contribution
follows the docs guidelines:
https://nextjs.org/docs/community/contribution-guide
Co-authored-by: Zack Tanner <1939140+ztanner@users.noreply.github.com>
