Vary header CDN issues

[6TELOIV]

Similar to this pr and comment, we ran into issues with the Vary headers and our CDN. In particular, Akamai will skip caching entirely for any response including a Vary header besides Accept-Encoding. The presense of the _rsc param ensures that caches won't server INVALID responses, but the remaining presense of the Vary header at all causes incachability in this case.

Ideally, there would be a way in NextJS to disable the Vary headers, as they seem to have been superseeded by the _rsc query params and are not strictly necessary to be present in the response exiting the server. I see that they are used within some internal code for in-memory cache validation, so flat-out removing all references seems like a no-go. Instead, I think a flag in next.config is appropriate, which would simply remove the header at the last moment before sending a response.

Addtionally, it would be extremely helpful for these default headers to be documented at a minimum. I belive the CDN portion of self hosting may be an appropriate location: https://nextjs.org/docs/app/guides/self-hosting#usage-with-cdns