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/db/smallindex.go

@@ -8,7 +8,7 @@ package db
 
 import (
 	"encoding/binary"
-	"sort"
+	"slices"
 
 	"github.com/syncthing/syncthing/lib/db/backend"
 	"github.com/syncthing/syncthing/lib/sync"
@@ -147,6 +147,6 @@ func (i *smallIndex) Values() []string {
 	}
 	i.mut.Unlock()
 
-	sort.Strings(vals)
+	slices.Sort(vals)
 	return vals
 }