refactor: use slices package for sort (#10132)

The sort package is still used in places that were not trivial to
change. Since Go 1.21 slices package can be uswed for sort. See
https://go.dev/doc/go1.21#slices

### Purpose

Make some progress with the migration to a more up-to-date syntax.

lib/model/folder.go

@@ -12,6 +12,7 @@ import (
 	"fmt"
 	"math/rand"
 	"path/filepath"
+	"slices"
 	"sort"
 	"time"
 
@@ -1341,7 +1342,7 @@ func unifySubs(dirs []string, exists func(dir string) bool) []string {
 	if len(dirs) == 0 {
 		return nil
 	}
-	sort.Strings(dirs)
+	slices.Sort(dirs)
 	if dirs[0] == "" || dirs[0] == "." || dirs[0] == string(fs.PathSeparator) {
 		return nil
 	}

lib/model/model_test.go

@@ -18,7 +18,6 @@ import (
 	"path/filepath"
 	"runtime/pprof"
 	"slices"
-	"sort"
 	"strconv"
 	"strings"
 	"sync"
@@ -4092,8 +4091,8 @@ func equalStringsInAnyOrder(a, b []string) bool {
 	if len(a) != len(b) {
 		return false
 	}
-	sort.Strings(a)
-	sort.Strings(b)
+	slices.Sort(a)
+	slices.Sort(b)
 	for i := range a {
 		if a[i] != b[i] {
 			return false