refactor: replace golang.org/x/exp/slices with slices (#16772)

The experimental functions in `golang.org/x/exp/slices` are now
available in the standard library since Go 1.21.

Reference: https://go.dev/doc/go1.21#slices

Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

tailnet/node_internal_test.go

@@ -2,13 +2,13 @@ package tailnet
 
 import (
 	"net/netip"
+	"slices"
 	"testing"
 	"time"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"golang.org/x/exp/maps"
-	"golang.org/x/exp/slices"
 	"golang.org/x/xerrors"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/key"