document relation to Go 1.24's os.Root APIs

Go 1.24 introduces the `os.Root` family of file APIs: https://go.dev/blog/osroot (cc @neild). These APIs are resistant to path traversal.

If I'm understanding things correctly, this is a replacement for safeopen. It may be a good idea to:

1. Recommend users who can use Go 1.24+ to use `os.Root` instead of safeopen.
2. Reimplement safeopen on top of `os.Root`.
3. Deprecate (or mark with [inline me annotations](https://github.com/golang/go/issues/67049)) the safeopen functions to users can migrate once Go 1.24 is two releases old.