BUG: DatetimeIndex._data should return an ndarray
- [x] closes #20810
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

The change I made seems to fix the case in the original issue without breaking any tests. 

On my branch:
```
In [1]: idx1 = pd.DatetimeIndex(start="2012-01-01", periods=3, freq='D') # date_range kind of construction

In [2]: idx1._data
array(['2012-01-01T00:00:00.000000000', '2012-01-02T00:00:00.000000000',
       '2012-01-03T00:00:00.000000000'], dtype='datetime64[ns]')

In [3]: idx2 = pd.DatetimeIndex(idx1)

In [4]: idx2._data
Out[4]: 
array(['2012-01-01T00:00:00.000000000', '2012-01-02T00:00:00.000000000',
       '2012-01-03T00:00:00.000000000'], dtype='datetime64[ns]')
```

But is the solution too simple or is something more sophisticated required?

And do we need tests for this issue?