minor #19972 [Serializer] Replace deprecated ISO8601 with Atom (andreybolonin)

This PR was submitted for the 7.1 branch but it was merged into the 5.4 branch instead.

Discussion
----------

[Serializer] Replace deprecated ISO8601 with Atom

<!--

If your pull request fixes a BUG, use the oldest maintained branch that contains
the bug (see https://symfony.com/releases for the list of maintained branches).

If your pull request documents a NEW FEATURE, use the same Symfony branch where
the feature was introduced (and `7.x` for features of unreleased versions).

-->

Commits
-------

a21ac0c Update serializer.rst

Author: javiereguiluz

components/serializer.rst

@@ -799,7 +799,7 @@ When serializing, you can set a callback to format a specific object property::
 
     // all callback parameters are optional (you can omit the ones you don't use)
     $dateCallback = function ($innerObject, $outerObject, string $attributeName, ?string $format = null, array $context = []) {
-        return $innerObject instanceof \DateTime ? $innerObject->format(\DateTime::ISO8601) : '';
+        return $innerObject instanceof \DateTime ? $innerObject->format(\DateTime::ATOM) : '';
     };
 
     $defaultContext = [