Avoid deprecation warning in internal to_arrow code

In rapidsai#16297, we deprecated the use of `to_arrow` in favour of
`to_arrow_host` and `to_arrow_device`. However, the scalar detail
overload of `to_arrow` used the public table overload. So we get a
warning when compiling internal libcudf code. Fix this by using the
detail API, and fix a bug along the way where we were not passing
through the arrow memory resource.

cpp/src/interop/to_arrow.cu

@@ -458,7 +458,7 @@ std::shared_ptr<arrow::Scalar> to_arrow(cudf::scalar const& input,
 {
   auto const column = cudf::make_column_from_scalar(input, 1, stream);
   cudf::table_view const tv{{column->view()}};
-  auto const arrow_table  = cudf::to_arrow(tv, {metadata}, stream);
+  auto const arrow_table  = detail::to_arrow(tv, {metadata}, stream, ar_mr);
   auto const ac           = arrow_table->column(0);
   auto const maybe_scalar = ac->GetScalar(0);
   if (!maybe_scalar.ok()) { CUDF_FAIL("Failed to produce a scalar"); }