Use `flox` for median-aggregation in groupbys

[sdiebolt]

Is your feature request related to a problem?

If I'm not mistaken, flox does support median/nanmedian aggregation for groupby reduce operations. But currently, while Xarray's docs indicates that median reduction on groupbys should use flox when available, I'm not sure this is actually implemented:

xarray/xarray/core/_aggregations.py

Lines 7807 to 7813 in 885e8d1

	return self.reduce(
	duck_array_ops.median,
	dim=dim,
	skipna=skipna,
	keep_attrs=keep_attrs,
	**kwargs,
	)

I'm happy to open a PR if median-reductions should indeed use flox when available!

Describe the solution you'd like

Median-reduction on groupbys should use flox when available.

Describe alternatives you've considered

No response

Additional context

Currently, I resort to using flox's xarray_reduce function directly, which dramatically speeds up median-reduction.

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