ContentLoader: Fix updating spouts on PHP < 7.2

The constants were only introduced in 7.2.0.

https://www.php.net/manual/en/class.datetimeinterface.php#datetimeinterface.changelog

src/helpers/ContentLoader.php

@@ -167,7 +167,7 @@ public function fetch($source) {
                     $itemDate = new \DateTimeImmutable();
                 }
                 if ($itemDate < $minDate) {
-                    $this->logger->debug('item "' . $item->getTitle() . '" (' . $itemDate->format(\DateTimeInterface::ATOM) . ') older than ' . $this->configuration->itemsLifetime . ' days');
+                    $this->logger->debug('item "' . $item->getTitle() . '" (' . $itemDate->format(\DateTime::ATOM) . ') older than ' . $this->configuration->itemsLifetime . ' days');
                     continue;
                 }