Fix building doc following depreciation in Pandas 2.2.0

[nameloCmaS]

Fixes
./doc
make html

Following Pandas 2.2.0 release, pd.date_range freq arguments were depreciated.

Question why old releases in doc/whats-new.rst are compiled and not static to avoid breaking changes for old releases.

Error from building doc:

WARNING:
>>>-------------------------------------------------------------------------
Warning in /Users/sam/Documents/GitHub/xarray/doc/whats-new.rst at block ending on line 7231
Specify :okwarning: as an option in the ipython:: block to suppress this message
----------------------------------------------------------------------------
<ipython-input-92-be0529fe0129>:1: FutureWarning: 'M' is deprecated and will be removed in a future version, please use 'ME' instead.
  ds = xray.Dataset({"t": pd.date_range("2000-01-01", periods=12, freq="M")})
<<<-------------------------------------------------------------------------

Exception occurred:
  File "/Users/sam/miniconda3/envs/xarray-docs/lib/python3.10/site-packages/IPython/sphinxext/ipython_directive.py", line 602, in process_input
    raise RuntimeError(
RuntimeError: Unexpected warning in `/Users/sam/Documents/GitHub/xarray/doc/whats-new.rst` line 7231

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[nameloCmaS]

There are possibly 81 of these warnings. They will need fixing one day; do we start now? Pandas 2.2.0 release seems to be causing other pull requests to fail, possibly due to this issue also.

sam@R2D3 xarray % conda activate xarray-tests
(xarray-tests) sam@R2D3 xarray % python -m pytest --doctest-modules xarray --ignore xarray/tests -Werror
============================= test session starts ==============================
platform darwin -- Python 3.10.13, pytest-7.4.4, pluggy-1.3.0
rootdir: /Users/sam/Documents/GitHub/xarray
configfile: pyproject.toml
plugins: hypothesis-6.96.2, timeout-2.2.0, env-1.1.3, cov-4.1.0, xdist-3.5.0
collected 302 items                                                            

xarray/backends/api.py .F                                                [  0%]
xarray/backends/common.py ..                                             [  1%]
xarray/coding/cftime_offsets.py .                                        [  1%]
xarray/coding/cftimeindex.py ....                                        [  2%]
xarray/coding/strings.py .                                               [  3%]
xarray/coding/variables.py ..                                            [  3%]
xarray/core/_aggregations.py FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF [ 18%]
FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF                                      [ 29%]
xarray/core/accessor_dt.py ..F                                           [ 30%]
xarray/core/accessor_str.py .............................                [ 40%]
xarray/core/alignment.py ..                                              [ 41%]
xarray/core/combine.py ..                                                [ 41%]
xarray/core/common.py OMP: Info #276: omp_set_nested routine deprecated, please use omp_set_max_active_levels instead.
...........                                        [ 45%]
xarray/core/computation.py ......                                        [ 47%]
xarray/core/concat.py .                                                  [ 47%]
xarray/core/coordinates.py ..                                            [ 48%]
xarray/core/dataarray.py ..........F.................................... [ 63%]
.........                                                                [ 66%]
xarray/core/dataset.py .............................................     [ 81%]
xarray/core/extensions.py .                                              [ 82%]
xarray/core/groupby.py .                                                 [ 82%]
xarray/core/indexing.py ...                                              [ 83%]
xarray/core/merge.py .                                                   [ 83%]
xarray/core/options.py .                                                 [ 84%]
xarray/core/parallel.py .                                                [ 84%]
xarray/core/rolling.py ....                                              [ 85%]
xarray/core/rolling_exp.py ......                                        [ 87%]
xarray/core/utils.py .                                                   [ 88%]
xarray/core/variable.py ...                                              [ 89%]
xarray/namedarray/_aggregations.py .............                         [ 93%]
xarray/namedarray/_array_api.py ....                                     [ 94%]
xarray/namedarray/core.py .                                              [ 95%]
xarray/plot/dataarray_plot.py .                                          [ 95%]
xarray/plot/facetgrid.py ..                                              [ 96%]
xarray/plot/utils.py ..........                                          [ 99%]
xarray/testing/strategies.py ss                                          [100%]

=================================== FAILURES ===================================
_________________ [doctest] xarray.backends.api.save_mfdataset _________________
1424     compute : bool
1425         If true compute immediately, otherwise return a
1426         ``dask.delayed.Delayed`` object that can be computed later.
1427 
1428     Examples
1429     --------
1430 
1431     Save a dataset into one netCDF per year of data:
1432 
1433     >>> ds = xr.Dataset(
UNEXPECTED EXCEPTION: FutureWarning("'M' is deprecated and will be removed in a future version, please use 'ME' instead.")

/continues

[max-sixty]

Thanks for fixing that one. Could we add a TODO to the code and link back to this issue?

---

More generally:

Assuming we can't quickly fix the tests, I would vote to ignore them in the meantime, so we can have the other tests passing. This PR is an example of that approach. That also lowers the urgency on those who work on the cftime part of the package.

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