DEPR/API: disallow ffill/bfill method in "interpolate"

[jbrockmendel]

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Reproducible Example

ser = pd.Series([1, 2, None, 4, 5])

res = ser.interpolate(method="ffill")
assert res.equals(ser.ffill())

Issue Description

It looks like NDFrame.interpolate with a ffill/bfill "method" is redundant with ffill/bfill. interpolate also has limit_direction and limit_area keywords that could plausibly make it behave differently, need to track down whether that is the case. If it is, would it make more sense to have those keywords in ffill/bfill?

There should be One Obvious Way to do ffill/bfill, so support for these keyword values should be deprecated.

Expected Behavior

NA

Installed Versions

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