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GH-40023: [Python] Use Cast() instead of CastTo (#40116)



### Rationale for this change

Remove legacy code

### What changes are included in this PR?

* Replace the legacy scalar CastTo implementation for Python.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.

* Closes: #40023

Authored-by: Hyunseok Seo <hsseo0501@gmail.com>
Signed-off-by: AlenkaF <frim.alenka@gmail.com>