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GH-39225: [GLib] Use Cast() instaed of CastTo (#39228)



### Rationale for this change

Remove legacy code

### What changes are included in this PR?

* Replace the legacy scalar CastTo implementation for GLib. 

### Are these changes tested?

No.

### Are there any user-facing changes?

No.

* Closes: #39225

Authored-by: Hyunseok Seo <hsseo0501@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>