Unpin dask when slicing bug is fixed

[stephenworsley]

📰 Custom Issue

Dask is currently pinned by the bug raised in #6251, see dask/dask#11614. This pin should be removed when there is a version of dask which fixes this.

[trexfeathers]

Once un-pinned, we should stop skipping these two doctests, which are fixed by future releases of Dask:

iris/lib/iris/common/resolve.py

Lines 2595 to 2598 in 71fa550

	>>> Resolve(cube1, cube2).shape # doctest: +SKIP
	(240, 37, 49)
	>>> Resolve(cube2, cube1).shape # doctest: +SKIP
	(240, 37, 49)