internal/dl: require macos 11

Fixes golang/go#68891
For golang/go#64207
For golang/go#65614

Change-Id: I9256d9de0687b9e3cf674b4d318f4d5023a9bf16
Reviewed-on: https://go-review.googlesource.com/c/website/+/605875
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

internal/dl/dl.go

@@ -170,7 +170,7 @@ var featuredFiles = []Feature{
 	},
 	{
 		Platform:     "Apple macOS (x86-64)",
-		Requirements: "macOS 10.15 or later, Intel 64-bit processor",
+		Requirements: "macOS 11 or later, Intel 64-bit processor",
 		fileRE:       regexp.MustCompile(`\.darwin-amd64\.pkg$`),
 	},
 	{