Remove experimental namespace from make_strings_children

[davidwendt]

Description

Replaces the cudf::strings::detail::make_strings_children with the new cudf::strings::detail::experimental::make_strings_children. No code logic has changed -- just code moved around. All current code was already using the experimental function.

Checklist

• [x] I am familiar with the Contributing Guidelines.
• [x] New or existing tests cover these changes.
• [x] The documentation is up to date with these changes.