fix: Set shrink_dtype output type to Unknown

[kdn36]

Partial fix for #23663.

Request review focus on deprecation changes, including scope (series.py vs expr.py, eager vs lazy).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.38%. Comparing base (8a55acc) to head (417b223).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23793      +/-   ##
==========================================
+ Coverage   81.34%   81.38%   +0.03%     
==========================================
  Files        1651     1652       +1     
  Lines      224251   224336      +85     
  Branches     2866     2872       +6     
==========================================
+ Hits       182424   182569     +145     
+ Misses      41115    41048      -67     
- Partials      712      719       +7     

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