minor symfony#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
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[Serializer] Replace deprecated ISO8601 with Atom

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Commits
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a21ac0c Update serializer.rst

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 = [