Merge branch '6.4' into 7.0

* 6.4:
  [AssetMapper] Allow simple, relative paths in importmap.php
  [PropertyInfo] Make isWriteable() more consistent with isReadable() when checking snake_case properties
  [AssetMapper] Add support for CSS files in the importmap
  [Messenger] Add `--all` option to the `messenger:failed:remove` command
  Fix merge
  fix #51235 - fix the order of merging of serializationContext and self::CONTEXT_DENORMALIZE
  [HttpClient] Fix Static Code Analyzer issue with JsonMockResponse
  [Messenger] Fix exiting `FailedMessagesRetryCommand`
  [Serializer] Fix reindex normalizedData array in AbstractObjectNormalizer::denormalize()
  remove an unreachable code branch
  [Validator] Fix `File::$extensions`’ PHPDoc
  [Mime] Fix email (de)serialization issues
  [FrameworkBundle][WebProfilerBundle][Console][Form][HttpKernel][PropertyInfo][Validator] Remove optional before required param
  Replace usages of SkippedTestSuiteError with markTestSkipped() call
  Update InteractiveAuthenticatorInterface description wording to match documentation
  [Serializer] Fix parsing XML root node attributes
  fix parsing of payload timestamp to DateTimeImmutable
  Fix routing to multiple fallback transports
  Fix missing stamps in delayed message handling
  [DoctrineBridge] Ignore invalid stores in `LockStoreSchemaListener` raised by `StoreFactory`

Author: nicolas-grekas

Extension/DataCollector/FormDataCollector.php

@@ -259,7 +259,7 @@ private function &recursiveBuildPreliminaryFormTree(FormInterface $form, array &
         return $output;
     }
 
-    private function &recursiveBuildFinalFormTree(FormInterface $form = null, FormView $view, array &$outputByHash): array
+    private function &recursiveBuildFinalFormTree(?FormInterface $form, FormView $view, array &$outputByHash): array
     {
         $viewHash = spl_object_hash($view);
         $formHash = null;