silence zarr warnings

[d-v-b]

running the convert emits a lot of warnings like this from zarr python:

 UnstableSpecificationWarning: The data type (FixedLengthUTF32(length=7, endianness='little')) does not have a Zarr V3 specification. That means that the representation of arrays saved with this data type may change without warning in a future version of Zarr Python. Arrays stored with this data type may be unreadable by other Zarr libraries. Use this data type at your own risk! Check https://github.com/zarr-developers/zarr-extensions/tree/main/data-types for the status of data type specifications for Zarr V3.

a recently merged PR to zarr python makes it easier to silence these warnings specifically, without silencing warnings from other libraries. This feature will be in the next zarr python release, so whenever that's out, we should use it.