DEPR: downcast kwd in fillna

[jbrockmendel]

• closes DEPR: Deprecate downcast keyword for fillna #40988
• tests added / passed
• Ensure all linting tests pass, see here for how to run them
• whatsnew entry

I'm on the fence about this. downcast="infer" seems pretty useful, as evidenced by how often it shows up in the tests. We don't have anything analogous to infer_objects for "see if i can losslessly downcast numeric values".

If we go forward with this, probably need to deprecate within NDFrame.interpolate too, as apparently some but not all interpolate paths see this deprecation.

[jreback]

i'm on the fence about this. downcast="infer" seems pretty useful, as evidenced by how often it shows up in the tests

this was intended original to be a performance hit avoider, e.g. if the fillna ended up in object dtype we would automatically try to reconvert. but this seems silly as we do this elsewhere. so +1 on deprecating.

it was never intended to be a numeric downcaster (nor should it), we have to_numeric for that and it need to be explict

[jbrockmendel]

Mothballing pending re-thinking.