This PR updates docs to show v21 in the version picker. Currently, it shoulds v17, v18, v19, and v20. <img width="611" alt="Screenshot 2025-05-05 at 3 34 26 PM" src="https://github.com/user-attachments/assets/baee6f00-8db2-4300-ac80-944b8a6b760a" /> It should show v21, v20, and v19 according to our LTS policy.