Update to zarr-v3-compatible released version of Kerchunk (0.2.8)

[TomNicholas]

Long-awaited part of #392, as it allows us to pin zarr-python>=3!

cc @mpiannucci the warning in the icechunk docs can be updated now (though virtualizarr still needs to be fixed to work with the latest version of icechunk).

• Closes part of Planning explicit dependency on Zarr v3 #392
• Tests added
• Tests passing
• Full type hint coverage
• Changes are documented in docs/releases.rst
• New functions/methods are listed in api.rst
• New functionality has documentation

[TomNicholas]

We need kerchunk to be on conda for the CI to work https://github.com/conda-forge/kerchunk-feedstock @martindurant

[martindurant]

The bot should pick it up later today.

[martindurant]

conda forge PR is merged, artefact should be available soon.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.17%. Comparing base (83a6f10) to head (eb494cf).
Report is 32 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #412      +/-   ##
==========================================
+ Coverage   77.81%   84.17%   +6.36%     
==========================================
  Files          31       31              
  Lines        1839     1839              
==========================================
+ Hits         1431     1548     +117     
+ Misses        408      291     -117     

see 8 files with indirect coverage changes

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[TomNicholas]

Only failure is an icechunk one which also occurs on main (and I've mentioned it upstream earth-mover/icechunk#665 (comment)) so I will merge this now.

Thank so much @martindurant !