Revert the value of MAX_BATCH_OPERATION_SIZE in gcsio

[shunping]

This value was changed in PR #29360 to 1000, which led to internal test failure.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (32b1067) 37.85% compared to head (374105f) 37.85%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #29503   +/-   ##
=======================================
  Coverage   37.85%   37.85%           
=======================================
  Files         690      690           
  Lines      101306   101306           
=======================================
  Hits        38350    38350           
  Misses      61363    61363           
  Partials     1593     1593           

Flag	Coverage Δ
python	29.01% <100.00%> (ø)

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[johnjcasey]

LGTM