Update zarr in SPEC 0

[dstansby]

SPEC 0 has a nice chart illustrating when projects should drop scientific python dependencies: https://scientific-python.org/specs/spec-0000/#support-window. This includes zarr, but is out of date - at least it doesn't have v3 in the figure, and I'm guessing some of the dates from v2 might need checking too.

[joshmoore]

Thanks, @dstansby. Definitely for keeping this aligned with the zarr-python team's intents. We might ask for guidance for the scientific python folks if, e.g., we wanted to shorten the 2.18.x line since the statement has been in the wild for a while.