minor #51150 Use Stringable interface as much as possible (VincentLanglet)

This PR was squashed before being merged into the 6.4 branch.

Discussion
----------

Use Stringable interface as much as possible

| Q             | A
| ------------- | ---
| Branch?       | 6.4
| Bug fix?      | no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tickets       | Fix #... <!-- prefix each issue number with "Fix #", no need to create an issue if none exists, explain below instead -->
| License       | MIT
| Doc PR        | symfony/symfony-docs#... <!-- required for new features -->

Instead of requiring `__toString()` in interfaces, wouldn't it be better to extend `\Stringable` ?
(I kept the method when comment was added)

Commits
-------

43a7704636 Use Stringable interface as much as possible

Author: fabpot

FormErrorIterator.php

@@ -35,7 +35,7 @@
  * @implements \RecursiveIterator<int, T>
  * @implements \SeekableIterator<int, T>
  */
-class FormErrorIterator implements \RecursiveIterator, \SeekableIterator, \ArrayAccess, \Countable
+class FormErrorIterator implements \RecursiveIterator, \SeekableIterator, \ArrayAccess, \Countable, \Stringable
 {
     /**
      * The prefix used for indenting nested error messages.