dl: add go1.21rc3

Change-Id: Ia00547841a150ffc3274ae5fb08526729de6075d
Reviewed-on: https://go-review.googlesource.com/c/dl/+/509755
Reviewed-by: Eli Bendersky <eliben@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Gopher Robot <gobot@golang.org>
Run-TryBot: Gopher Robot <gobot@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

go1.21rc3/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 go1.21rc3 command runs the go command from Go 1.21rc3.
+//
+// To install, run:
+//
+//	$ go install golang.org/dl/go1.21rc3@latest
+//	$ go1.21rc3 download
+//
+// And then use the go1.21rc3 command as if it were your normal go
+// command.
+//
+// See the release notes at https://tip.golang.org/doc/go1.21.
+//
+// File bugs at https://go.dev/issue/new.
+package main
+
+import "golang.org/dl/internal/version"
+
+func main() {
+	version.Run("go1.21rc3")
+}