all: require Go 1.21

Updates #8419

Change-Id: I809b6a4d59d92a2ab6ec587ccbb9053376bf02c2
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscaled/required_version.go

@@ -1,10 +1,10 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
-//go:build !go1.20
+//go:build !go1.21
 
 package main
 
 func init() {
-	you_need_Go_1_20_to_compile_Tailscale()
+	you_need_Go_1_21_to_compile_Tailscale()
 }