[release-23.0] copy_state: use a mediumblob instead of a smaller varbinary for lastpk (#18852)

[vitess-bot]

Description

This is a backport of #18852

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.70%. Comparing base (bee41a6) to head (e2f85d2).
⚠️ Report is 3 commits behind head on release-23.0.

Additional details and impacted files

@@              Coverage Diff              @@
##           release-23.0   #18859   +/-   ##
=============================================
  Coverage         69.70%   69.70%           
=============================================
  Files              1606     1606           
  Lines            214482   214482           
=============================================
+ Hits             149494   149501    +7     
+ Misses            64988    64981    -7     

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