all: use Go 1.21 slices, maps instead of x/exp/{slices,maps}

Updates #8419

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

util/multierr/multierr.go

@@ -7,9 +7,8 @@ package multierr
 
 import (
 	"errors"
+	"slices"
 	"strings"
-
-	"golang.org/x/exp/slices"
 )
 
 // An Error represents multiple errors.

util/set/slice.go

@@ -4,7 +4,8 @@
 package set
 
 import (
-	"golang.org/x/exp/slices"
+	"slices"
+
 	"tailscale.com/types/views"
 )
 

util/slicesx/slicesx_test.go

@@ -5,9 +5,8 @@ package slicesx
 
 import (
 	"reflect"
+	"slices"
 	"testing"
-
-	"golang.org/x/exp/slices"
 )
 
 func TestInterleave(t *testing.T) {