DM-40150: Butler.datastore is no longer public attribute so use private one

[timj]

Checklist

• added documentation for a new migration script

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7ce1343) 54.37% compared to head (bdfe3fa) 54.37%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #30   +/-   ##
=======================================
  Coverage   54.37%   54.37%           
=======================================
  Files          33       33           
  Lines        1144     1144           
  Branches      256      256           
=======================================
  Hits          622      622           
  Misses        488      488           
  Partials       34       34           

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