internal/dl: update minimum macOS version to 10.13

Featured downloads on golang.org/dl use the latest Go release, which
is now Go 1.17. Per golang.org/doc/go1.17#darwin, Go 1.17 requires
macOS 10.13 High Sierra or later.

For golang/go#23011.
Fixes golang/go#47867.

Change-Id: I868d118f073977c74429d70ae9ab7c526d098cf7
Reviewed-on: https://go-review.googlesource.com/c/website/+/344071
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Paul Jolly <paul@myitcv.org.uk>
diff --git a/internal/dl/dl.go b/internal/dl/dl.go
index 66ad690..aaedf0a 100644
--- a/internal/dl/dl.go
+++ b/internal/dl/dl.go
@@ -161,7 +161,7 @@
 	},
 	{
 		Platform:     "Apple macOS",
-		Requirements: "macOS 10.12 or later, Intel 64-bit processor",
+		Requirements: "macOS 10.13 or later, Intel 64-bit processor",
 		fileRE:       regexp.MustCompile(`\.darwin-amd64\.pkg$`),
 	},
 	{