MinimumRequirements, Darwin: document that Go 1.27 requires macOS 13

For golang/go#75836.

Change-Id: Ida2bf9ee60cd9585d4071b675c4f369a93227cdc
Reviewed-on: https://go-review.googlesource.com/c/wiki/+/769800
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Sean Liao <sean@liao.dev>