Merge branch '6.4' into 7.0

* 6.4: (99 commits)
  [HttpClient][Messenger] add `PingWebhookMessage` and `PingWebhookMessageHandler`
  Use Stringable interface as much as possible
  [Messenger][Process] add `RunProcessMessage` and `RunProcessMessageHandler`
  Add generics to the progress bar iteration methods
  [SecurityBundle] Allow an array of `pattern` in firewall configuration
  [FrameworkBundle] Simplify marking store configuration
  [PsrHttpMessageBridge] Patch return types and fix CS
  [PsrHttpMessageBridge] Import the bridge into the monorepo
  Prepare release 2.3.1
  Fix CS
  Don't rely on Request::getPayload() to populate the parsed body
  Prepare release 2.3.0
  Implement ValueResolverInterface
  Leverage `Request::getPayload()` to populate the parsed body of PSR-7 requests
  Add native types where possible
  cs fix
  Prepare the 2.2.0 release
  Bump psr/http-message version
  Drop support for Symfony 4
  Adjustments for PHP CS Fixer 3
  ...

Author: derrabus

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.