dl: add go1.21.0

Change-Id: I53396b66d60a8b627412a1b3b82a435c4bfec9cc
Reviewed-on: https://go-review.googlesource.com/c/dl/+/516716
Run-TryBot: Gopher Robot <gobot@golang.org>
Auto-Submit: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
diff --git a/go1.21.0/main.go b/go1.21.0/main.go
new file mode 100644
index 0000000..3f48c64
--- /dev/null
+++ b/go1.21.0/main.go
@@ -0,0 +1,24 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// The go1.21.0 command runs the go command from Go 1.21.0.
+//
+// To install, run:
+//
+//	$ go install golang.org/dl/go1.21.0@latest
+//	$ go1.21.0 download
+//
+// And then use the go1.21.0 command as if it were your normal go
+// command.
+//
+// See the release notes at https://go.dev/doc/go1.21.
+//
+// File bugs at https://go.dev/issue/new.
+package main
+
+import "golang.org/dl/internal/version"
+
+func main() {
+	version.Run("go1.21.0")
+}