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.

cmd/syncthing/main.go

@@ -23,7 +23,7 @@ import (
 	"path/filepath"
 	"regexp"
 	"runtime/pprof"
-	"sort"
+	"slices"
 	"strconv"
 	"strings"
 	"syscall"
@@ -443,7 +443,7 @@ func debugFacilities() string {
 			maxLen = len(name)
 		}
 	}
-	sort.Strings(names)
+	slices.Sort(names)
 
 	// Format the choices
 	b := new(bytes.Buffer)