Deprecate partial read/writes in v2

[dstansby]

Since this was deprecated in numcodecs (zarr-developers/numcodecs#619), deprecate it here too.

I think it would be good to put this in 2.18.5, then remove the code in 2.19.0 to reduce the maintenance burden on zarr-python v2. Any thoughts on that?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.95%. Comparing base (2bf7e45) to head (682ca5e).
Report is 19 commits behind head on support/v2.

Additional details and impacted files

@@             Coverage Diff             @@
##           support/v2    #2844   +/-   ##
===========================================
  Coverage       99.95%   99.95%           
===========================================
  Files              38       38           
  Lines           14672    14680    +8     
===========================================
+ Hits            14666    14674    +8     
  Misses              6        6           

Files with missing lines	Coverage Δ
zarr/core.py	100.00% <100.00%> (ø)
zarr/creation.py	100.00% <ø> (ø)
zarr/tests/test_core.py	99.94% <100.00%> (+<0.01%)	⬆️

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