DEPR: DatetimeIndex.offset
`DatetimeIndex.offset` and `DatimeIndex.freq` refer to the same thing, and are used interchangeably throughout the code for `DatetimeIndex`:

https://github.com/pandas-dev/pandas/blob/4a344972722cc3c27250cbc8e382472b13e66bde/pandas/core/indexes/datetimes.py#L1729-L1732

Is there a reason for having both?  

`TimedeltaIndex` and `PeriodIndex` only have `freq`, so it seems like we could keep `freq` for consistency, deprecate `DatetimeIndex.offset`, and modify the `DatetimeIndex` code to only use `freq`.