all: switch to and require Go 1.23

Updates #12912

Change-Id: Ib4ae26eb5fb68ad2216cab4913811b94f7eed5b6
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 !go1.21
+//go:build !go1.23
 
 package main
 
 func init() {
-	you_need_Go_1_21_to_compile_Tailscale()
+	you_need_Go_1_23_to_compile_Tailscale()
 }