Fix missing Redoc JS bundle

The /api-docs/ page is empty on my browser, and it will likely become empty for most people as their browser cache expires. The template was using redoc@next, which made sense at the time we implemented this because we needed recent fancy features from Redoc. But they are working on version 3.x, and released 3.0.0-rc0 which is now tagged as next.

drf-spectacular happens to have its own configuration for the Redoc URL, which is currently using redoc@latest, so I am using this instead so that we can just upgrade Spectacular instead of having to worry about Redoc releases directly.