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

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

Discussion
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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
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43a7704 Use Stringable interface as much as possible

Author: fabpot

HeaderBag.php

@@ -18,7 +18,7 @@
  *
  * @implements \IteratorAggregate<string, list<string|null>>
  */
-class HeaderBag implements \IteratorAggregate, \Countable
+class HeaderBag implements \IteratorAggregate, \Countable, \Stringable
 {
     protected const UPPER = '_ABCDEFGHIJKLMNOPQRSTUVWXYZ';
     protected const LOWER = '-abcdefghijklmnopqrstuvwxyz';