refactor: use slices package for sorting (#10136)

Few more complicated usages of the sort packages are left.

### Purpose

Make progress towards replacing the sort package with slices package.

lib/model/folder_recvonly.go

@@ -7,7 +7,8 @@
 package model
 
 import (
-	"sort"
+	"slices"
+	"strings"
 	"time"
 
 	"github.com/syncthing/syncthing/lib/config"
@@ -207,7 +208,9 @@ func (q *deleteQueue) handle(fi protocol.FileInfo, snap *db.Snapshot) (bool, err
 
 func (q *deleteQueue) flush(snap *db.Snapshot) ([]string, error) {
 	// Process directories from the leaves inward.
-	sort.Sort(sort.Reverse(sort.StringSlice(q.dirs)))
+	slices.SortFunc(q.dirs, func(a, b string) int {
+		return strings.Compare(b, a)
+	})
 
 	var firstError error
 	var deleted []string