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>

pty/ptytest/ptytest.go

@@ -8,6 +8,7 @@ import (
 	"io"
 	"regexp"
 	"runtime"
+	"slices"
 	"strings"
 	"sync"
 	"testing"
@@ -16,7 +17,6 @@ import (
 
 	"github.com/acarl005/stripansi"
 	"github.com/stretchr/testify/require"
-	"golang.org/x/exp/slices"
 	"golang.org/x/xerrors"
 
 	"github.com/coder/coder/v2/pty"