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>

site/site.go

@@ -19,6 +19,7 @@ import (
 	"os"
 	"path"
 	"path/filepath"
+	"slices"
 	"strings"
 	"sync"
 	"sync/atomic"
@@ -29,7 +30,6 @@ import (
 	"github.com/justinas/nosurf"
 	"github.com/klauspost/compress/zstd"
 	"github.com/unrolled/secure"
-	"golang.org/x/exp/slices"
 	"golang.org/x/sync/errgroup"
 	"golang.org/x/sync/singleflight"
 	"golang.org/x/xerrors"