Use Zarr v3 best practises and remove warnings

[jeromekelleher]

We're getting a lot of warnings currently. Would be good to follow what has hopefully settled as being best practices for v3 and get rid of them.

====================================== warnings summary ======================================
tests/test_tskit.py: 96 warnings
  /home/jk/work/github/bio2zarr/bio2zarr/zarr_utils.py:47: ZarrDeprecationWarning: Use Group.create_array instead.
    array = group.array(                                                                      
                                                                                              
tests/test_tskit.py: 18 warnings                                                              
  /home/jk/work/github/bio2zarr/.venv/lib/python3.12/site-packages/zarr/core/group.py:3108: ZarrUserWarning: The `compressor` argument is deprecated. Use `compressors` instead.
    compressors = _parse_deprecated_compressor(compressor, compressors)
                                                                                              
tests/test_tskit.py: 96 warnings                                                              
  /home/jk/work/github/bio2zarr/.venv/lib/python3.12/site-packages/zarr/core/group.py:3111: ZarrDeprecationWarning: Use AsyncGroup.create_array instead.
    self._async_group.create_dataset(