[Serializer] Fix workaround min php version

Author: ogizanagi

Normalizer/DateTimeNormalizer.php

@@ -81,7 +81,7 @@ public function denormalize($data, $class, $format = null, array $context = arra
         $timezone = $this->getTimezone($context);
 
         if (null !== $dateTimeFormat) {
-            if (null === $timezone && PHP_VERSION_ID < 50600) {
+            if (null === $timezone && PHP_VERSION_ID < 70000) {
                 // https://bugs.php.net/bug.php?id=68669
                 $object = \DateTime::class === $class ? \DateTime::createFromFormat($dateTimeFormat, $data) : \DateTimeImmutable::createFromFormat($dateTimeFormat, $data);
             } else {