[DEP] Move impute to legacy

[TonyBagnall]

scheduled dep

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I would have added the following labels to this PR based on the changes made: [ $\color{#BCAE15}{\textsf{classification}}$, $\color{#31FCCD}{\textsf{forecasting}}$, $\color{#41A8F6}{\textsf{transformations}}$ ], however some package labels are already present.

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