DEPR: Enforce deprecation of previous implementation of DataFrame.stack
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Ref: https://github.com/pandas-dev/pandas/issues/53515

We need to keep the `future_stack` argument in 3.0, but have two choices: still allow users to specify `future_stack=False` (while always seeing the warning message) or remove `future_stack=False` entirely now. In either case, we'd introduce a deprecation of the `future_stack` argument (regardless of how it's being specified) as part of 3.x (I plan on 3.1). 

I went with the former option of still allowing `future_stack=False` - it seemed more gradual.
> We need to keep the `future_stack` argument in 3.0

that's unfortunate. Also were you thinking of deprecating `dropna` and `sort` in this PR as well? > Also were you thinking of deprecating `dropna` and `sort` in this PR as well?

Currently if you use `future_stack=True` we raise when dropna or sort are specified. So I don't think any action needs to be taken here. OK but in a follow up they will be deprecated then? I see from the 2.1 whatsnew

> As such, the arguments dropna and sort are not utilized and must remain unspecified when using future_stack=True. These arguments will be removed in the next major release. I don't think there is a need to deprecate them - they must remain unspecified and so removing them is backwards compatible. In other words, if you run pandas 2.x (or 3.x) without any warnings, then your code will be fine in 4.0. > So in 4.0 we can just remove `sort` and `dropna` from the signature of `stack`?

Correct. Thanks @rhshadrach 