DEPR: Timestamp.freq
- [x] closes #15146
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

This in turn will allow us to remove DatetimeArray.freq and TimedeltaArray.freq, which is needed for #31218