Use ME in test_plot instead of M (#9035)

```
pytest xarray/tests/test_plot.py::TestNcAxisNotInstalled::test_ncaxis_notinstalled_line_plot
```

would return the following error

```
xarray/tests/test_plot.py E                                                    [100%]

======================================= ERRORS =======================================
____ ERROR at setup of TestNcAxisNotInstalled.test_ncaxis_notinstalled_line_plot _____

self = <xarray.tests.test_plot.TestNcAxisNotInstalled object at 0x78ed1992aa10>

    @pytest.fixture(autouse=True)
    def setUp(self) -> None:
        """
        Create a DataArray with a time-axis that contains cftime.datetime
        objects.
        """
        month = np.arange(1, 13, 1)
        data = np.sin(2 * np.pi * month / 12.0)
        darray = DataArray(data, dims=["time"])
>       darray.coords["time"] = xr.cftime_range(
            start="2017", periods=12, freq="1M", calendar="noleap"
        )

/home/mark/git/xarray/xarray/tests/test_plot.py:3004:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
/home/mark/git/xarray/xarray/coding/cftime_offsets.py:1129: in cftime_range
    offset = to_offset(freq)
/home/mark/git/xarray/xarray/coding/cftime_offsets.py:767: in to_offset
    _emit_freq_deprecation_warning(freq)
/home/mark/git/xarray/xarray/coding/cftime_offsets.py:751: in _emit_freq_deprecation_warning
    emit_user_level_warning(message, FutureWarning)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

message = "'M' is deprecated and will be removed in a future version. Please use 'ME' instead of 'M'."
category = <class 'FutureWarning'>

    def emit_user_level_warning(message, category=None) -> None:
        """Emit a warning at the user level by inspecting the stack trace."""
        stacklevel = find_stack_level()
>       return warnings.warn(message, category=category, stacklevel=stacklevel)
E       FutureWarning: 'M' is deprecated and will be removed in a future version. Please use 'ME' instead of 'M'.

/home/mark/git/xarray/xarray/core/utils.py:1112: FutureWarning
============================== short test summary info ===============================
ERROR xarray/tests/test_plot.py::TestNcAxisNotInstalled::test_ncaxis_notinstalled_line_plot - FutureWarning: 'M' is deprecated and will be removed in a future version. Please ...
================================== 1 error in 0.64s ==================================
```

xarray/tests/test_plot.py

@@ -3002,7 +3002,7 @@ def setUp(self) -> None:
         data = np.sin(2 * np.pi * month / 12.0)
         darray = DataArray(data, dims=["time"])
         darray.coords["time"] = xr.cftime_range(
-            start="2017", periods=12, freq="1M", calendar="noleap"
+            start="2017", periods=12, freq="1ME", calendar="noleap"
         )
 
         self.darray = darray