Deprecate legacy DataFrame implementation

[phofl]

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We haven't done anything on the old implementation in a while and bugs are only fixed for the new one. I.e. the old version is not compatible with newer pandas releases and will break horribly with pandas 3 for example.

I think we should deprecate the mode and remove in a few releases. This would also making merging the repositories a lot easier since we don't have to content with the old implementation.

A real pain at the moment is that you still end up at the old API docs if you google things like

dask read_parquet, this is because the path to the docs didn't change (it is built based on the path of the function definition9

Thoughts?

[fjetter]

+1 Let's get a warning out asap. When we'll drop the actual implementation is something we can discuss further but the decision has been made a long time ago to get rid of the old implementation and as you said, it is effectively already in sunset so let's be loud about it.