io & os (replacing ioutil)

The io/ioutil package was deprecated in Go 1.16. Its contents weren't deleted, they were redistributed: stream helpers moved to io, and filesystem helpers moved to os. New code should call the new names, and existing code is easy to migrate because every old function maps to exactly one new function. This lesson walks the full migration map with side-by-side examples, explains the one subtle behavior change (os.ReadDir returns fs.DirEntry, not os.FileInfo), and shows how to find and rewrite ioutil calls automatically.