cmd/tailscaled: make build fail nicely on older Go versions

Due to a bug in Go (golang/go#51778), cmd/go doesn't warn about your
Go version being older than the go.mod's declared Go version in that
case that package loading fails before the build starts, such as when
you use packages that are only in the current version of Go, like our
use of net/netip.

This change works around that Go bug by adding build tags and a
pre-Go1.18-only file that will cause Go 1.17 and earlier to fail like:

    $ ~/sdk/go1.17/bin/go install ./cmd/tailscaled
    # tailscale.com/cmd/tailscaled
    ./required_version.go:11:2: undefined: you_need_Go_1_18_to_compile_Tailscale
    note: module requires Go 1.18

Change-Id: I39f5820de646703e19dde448dd86a7022252f75c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscaled/debug.go

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build go1.18
+// +build go1.18
+
 package main
 
 import (