feat(python): Improve internal string → date casts in read_excel

[alexander-beedie]

Closes #20832.

• Ensures that Date casts indicated by schema_overrides succeed by having fastexcel read them as datetimes before we apply the override casts (so that date strings with trailing " 00:00:00" time components do not fail).
• Note that we do not generate the other warnings mentioned in the issue - that is an upstream issue, so we can close it out here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.63%. Comparing base (9c1637c) to head (abd598f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20845      +/-   ##
==========================================
- Coverage   79.63%   79.63%   -0.01%     
==========================================
  Files        1571     1571              
  Lines      223146   223146              
  Branches     2546     2546              
==========================================
- Hits       177711   177709       -2     
- Misses      44851    44853       +2     
  Partials      584      584              

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