rename seed to cache_seed

[sonichi]

openai introduces a new param in chat completion api:

• seed: This feature is in Beta. If specified, our system will make a best effort to
  sample deterministically, such that repeated requests with the same seed and
  parameters should return the same result. Determinism is not guaranteed, and you
  should refer to the system_fingerprint response parameter to monitor changes
  in the backend.

"Determinism is not guaranteed" - that's a reason to keep the seed in autogen, but rename it to cache_seed. Then, we'll be able to support both openai seed and autogen seed.

### Tasks
- [x] seed -> cache_seed in code + doc
- [x] add this change to migration guide
- [ ] seed -> cache_seed in notebooks & blogposts for retrieve chat, teachable agent, mathchat

[jackgerrits]

@sonichi can this be closed now?

[ekzhu]

Yes.

[sonichi]

mathchat still depends on the old version of pyautogen. @kevin666aa may need to decide what to do with it.

[yiranwu0]

mathchat still depends on the old version of pyautogen. @kevin666aa may need to decide what to do with it.

Got it. I will go over MathChat and propose a solution later this week.