Revert "AC-14431: Order items order_date wrong formatting"

This reverts commit b30aac8.

Author: glo60612

dev/tests/api-functional/testsuite/Magento/GraphQl/Sales/OrderDateFormattingTest.php

@@ -89,6 +89,7 @@ public function testOrderDateWithInvalidCreatedAt()
         $date = \DateTime::createFromFormat($expectedFormat, $dateString);
         $isValid = $date && $date->format($expectedFormat) === $dateString;
         $this->assertTrue($isValid, "Date format is not valid: $dateString");
+
     }
 
     /**

dev/tests/api-functional/testsuite/Magento/GraphQl/Sales/RetrieveOrdersByOrderNumberTest.php

@@ -11,6 +11,7 @@
 use Magento\Framework\Api\SearchCriteriaBuilder;
 use Magento\Framework\Exception\AuthenticationException;
 use Magento\Framework\Registry;
+use Magento\Framework\Stdlib\DateTime;
 use Magento\Framework\Stdlib\DateTime\TimezoneInterface;
 use Magento\GraphQl\GetCustomerAuthenticationHeader;
 use Magento\Sales\Api\OrderRepositoryInterface;
@@ -508,7 +509,7 @@ public function testGetCustomerDescendingSortedOrders()
         for ($i = 1; $i <= 3; $i++) {
             $orderNumber = $this->fixtures->get('or' . $i)->getIncrementId();
             $orderCreatedAt = $this->timezone->date($this->fixtures->get('or' . $i)->getCreatedAt())
-                ->format('d/m/Y H:i:s');
+                ->format(DateTime::DATETIME_PHP_FORMAT);
             $orderNumberCreatedAtExpected[$orderNumber] = $orderCreatedAt;
         }