Consider removing `allow_missing` in update requests in favor of Apply/PUT

[rofrankel]

          this is a very ugly pattern - we should remove it in favor of PUT / Apply. The logic to implement this on the server gets pretty complicated too (since you have to consider errors that are encountered with create as well as update).

I'd prefer to just remove this guidance now if we agree, and I'll come back with the PUT AEP. But if we don't agree I can just file an issue.

Originally posted by @toumorokoshi in https://github.com/aep-dev/aep.dev/pull/115#discussion_r1522489105