delete! in DataFrames.jl

[bkamins]

We currently have delete! to remove rows from a DataFrame. However, delete! is defined for collections in Julia Base REMOVING A MAPPING for a passed key. Clearly this is not what delete! in DataFrames.jl does.

I propose to:

• deprecate delete! (it is a long term deprecation - it does not hurt to keep it)
• what delete! does should be deleteat!
• additionally implement keepat! to match Julia 1.7 behavior

(the issue is prompted by the fact that when we add keepat! it is strange to have delete! as its counterpart)

CC @nalimilan and @oxinabox (for end-user opinion as usual 😄)

[nalimilan]

For reference, I introduced delete! at #2189 but it should clearly have been called deleteat!.