[release-branch.go1.22] spec: clarify iteration variable type for range over integer

[release-branch.go1.22] spec: clarify iteration variable type for range over integer

Also: report language version (plus date) in spec header.

For #65137.

Change-Id: I4f1d220d5922c40a36264df2d0a7bb7cd0756bac
Reviewed-on: https://go-review.googlesource.com/c/go/+/557596
TryBot-Bypass: Robert Griesemer <gri@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/559875