document relation to Go 1.24's os.Root APIs

[aktau]

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) the safeopen functions to users can migrate once Go 1.24 is two releases old.

[neild]

A few notes on safeopen vs. os.Root:

• os.Root supports a few more platforms than safeopen. (Notably GOOS=wasip1, probably less notably GOOS=plan9.)
• os.Root doesn't use openat2/RESOLVE_BENEATH yet (hopefully it will in Go 1.25) and will be less efficient than safeopen when openat2 is available.
• os.Root tries quite hard to preserve platform path semantics: For example, on Unix a/../b follows (non-escaping) symlinks in a. I believe safeopen rejects paths containing .. components. (From a quick look, safeopen also rejects symlinks entirely on systems that don't have openat2/RESOLVE_BENEATH?)