Refactor POPULAR_BABY_NAMES_2021 to use Rc<RefCell> instead of Arc<Mutex>

[DannyCeb]

Summary

This PR refactors the POPULAR_BABY_NAMES_2021 global variable to use Rc<RefCell<Vec<String>>> instead of Arc<Mutex<Option<Vec<String>>>>. Since the data is only accessed within a single thread, this change removes unnecessary synchronization overhead while preserving interior mutability.

Rationale

• Rc<RefCell<T>> is more appropriate for single-threaded scenarios.
• Simplifies the code and improves readability.
• Keeps the behavior consistent with the original intent.

Notes

Most of the file was left untouched to maintain alignment with the book version.

[DannyCeb]

Let me know if a comment in main.rs would be helpful to clarify this change as an errata or deviation.

[brndnmtthws]

The build error appears unrelated to your changes, and I fixed it in #26. If you merge or rebase against main, the build should pass now.