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

Authentication/Token/TokenInterface.php

@@ -19,7 +19,7 @@
  * @author Fabien Potencier <[email protected]>
  * @author Johannes M. Schmitt <[email protected]>
  */
-interface TokenInterface
+interface TokenInterface extends \Stringable
 {
     /**
      * Returns a string representation of the Token.

User/InMemoryUser.php

@@ -19,7 +19,7 @@
  * @author Robin Chalas <[email protected]>
  * @author Fabien Potencier <[email protected]>
  */
-final class InMemoryUser implements UserInterface, PasswordAuthenticatedUserInterface, EquatableInterface
+final class InMemoryUser implements UserInterface, PasswordAuthenticatedUserInterface, EquatableInterface, \Stringable
 {
     private string $username;
     private ?string $password;