internal/dl: update minimum macOS version to 10.12

Featured downloads on golang.org/dl use the latest Go release, which
is now Go 1.15. Per golang.org/doc/go1.15#darwin, Go 1.15 requires
macOS 10.12 Sierra or later.

For golang/go#23011.

Change-Id: I9e1fabe459d74bf59661d84e4b40d39d29ce2638
Reviewed-on: https://go-review.googlesource.com/c/website/+/234522
Reviewed-by: Carlos Amedee <[email protected]>

Author: dmitshur

internal/dl/dl.go

@@ -161,7 +161,7 @@ var featuredFiles = []Feature{
 	},
 	{
 		Platform:     "Apple macOS",
-		Requirements: "macOS 10.11 or later, Intel 64-bit processor",
+		Requirements: "macOS 10.12 or later, Intel 64-bit processor",
 		fileRE:       regexp.MustCompile(`\.darwin-amd64(-osx10\.8)?\.pkg$`),
 	},
 	{