Update "Upgrade" link to correct docs page

[nataliekwong]

Currently, the "Upgrade" button link (in the left-hand sidebar) points to docs.airbyte.io/upgrading-airbyte, which is a dead link.
The correct link is: https://docs.airbyte.com/operator-guides/upgrading-airbyte/

The destination URL should be changed.

If possible, the original URL should redirect as well to the new URL, so that old versions of Airbyte navigate to the correct page.

[octavia-squidington-iii]

cc @airbytehq/frontend