Avoid pandas warning

[dopplershift]

Apparently with Pandas 2.2.0 (tested with the just released rc0) freq='H' is deprecated, in favor of freq='h'. Since this works across our versions, go ahead and change.

Otherwise we see:

=================================== FAILURES ===================================
_______________________ test_smooth_window_1d_dataarray ________________________

    def test_smooth_window_1d_dataarray():
        """Test smooth_window on 1D DataArray."""
        temperature = xr.DataArray(
            [37., 32., 34., 29., 28., 24., 26., 24., 27., 30.],
            dims=('time',),
>           coords={'time': pd.date_range('2020-01-01', periods=10, freq='H')},
            attrs={'units': 'degF'})

tests/calc/test_basic.py:712: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
/opt/hostedtoolcache/Python/3.12.1/x64/lib/python3.12/site-packages/pandas/core/indexes/datetimes.py:1008: in date_range
    dtarr = DatetimeArray._generate_range(
/opt/hostedtoolcache/Python/3.12.1/x64/lib/python3.12/site-packages/pandas/core/arrays/datetimes.py:424: in _generate_range
    freq = to_offset(freq)
offsets.pyx:4730: in pandas._libs.tslibs.offsets.to_offset
    ???
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

>   ???
E   FutureWarning: 'H' is deprecated and will be removed in a future version, please use 'h' instead.

offsets.pyx:4842: FutureWarning
=========================== short test summary info ============================
FAILED tests/calc/test_basic.py::test_smooth_window_1d_dataarray - FutureWarning: 'H' is deprecated and will be removed in a future version, please use 'h' instead.
============ 1 failed, 1510 passed, 1 xfailed in 467.89s (0:07:47) =============

(see current nightly failure in #3268)

Description Of Changes

Checklist

• Closes Nightly build is failing #3268