all: Use some Go 1.21 features (#9409)

lib/api/api_auth.go

@@ -11,6 +11,7 @@ import (
 	"fmt"
 	"net"
 	"net/http"
+	"slices"
 	"strings"
 	"time"
 
@@ -19,7 +20,6 @@ import (
 	"github.com/syncthing/syncthing/lib/db"
 	"github.com/syncthing/syncthing/lib/events"
 	"github.com/syncthing/syncthing/lib/rand"
-	"golang.org/x/exp/slices"
 )
 
 const (

lib/api/api_test.go

@@ -18,6 +18,7 @@ import (
 	"net/http/httptest"
 	"os"
 	"path/filepath"
+	"slices"
 	"strconv"
 	"strings"
 	"testing"
@@ -46,7 +47,6 @@ import (
 	"github.com/syncthing/syncthing/lib/tlsutil"
 	"github.com/syncthing/syncthing/lib/ur"
 	"github.com/thejerf/suture/v4"
-	"golang.org/x/exp/slices"
 )
 
 var (

lib/api/tokenmanager.go

@@ -7,12 +7,12 @@
 package api
 
 import (
+	"slices"
 	"time"
 
 	"github.com/syncthing/syncthing/lib/db"
 	"github.com/syncthing/syncthing/lib/rand"
 	"github.com/syncthing/syncthing/lib/sync"
-	"golang.org/x/exp/slices"
 )
 
 type tokenManager struct {