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>

c_glib/arrow-glib/scalar.cpp

@@ -26,6 +26,8 @@
 #include <arrow-glib/interval.hpp>
 #include <arrow-glib/scalar.hpp>
 
+#include <arrow/compute/cast.h>
+
 G_BEGIN_DECLS
 
 /**
@@ -385,9 +387,9 @@ garrow_scalar_cast(GArrowScalar *scalar,
 {
   const auto arrow_scalar = garrow_scalar_get_raw(scalar);
   const auto arrow_data_type = garrow_data_type_get_raw(data_type);
-  auto arrow_casted_scalar_result = arrow_scalar->CastTo(arrow_data_type);
+  auto arrow_casted_scalar_result = arrow::compute::Cast(arrow_scalar, arrow_data_type);
   if (garrow::check(error, arrow_casted_scalar_result, "[scalar][cast]")) {
-    auto arrow_casted_scalar = *arrow_casted_scalar_result;
+    auto arrow_casted_scalar = (*arrow_casted_scalar_result).scalar();
     return garrow_scalar_new_raw(&arrow_casted_scalar,
                                  "scalar", &arrow_casted_scalar,
                                  "data-type", data_type,