Fix DateTimeImmutable usage in Cookie.php

Ensure the global namespace is used for DateTimeImmutable to avoid potential namespace conflicts. This change improves code reliability and maintains consistency throughout the file.

src/Cookie.php

@@ -328,7 +328,7 @@ protected function normalize_attribute($name, $value) {
 					return null;
 				}
 
-				$expiry_time = ( new DateTimeImmutable( $value ) )->getTimestamp();
+				$expiry_time = ( new \DateTimeImmutable( $value ) )->getTimestamp();
 				if ($expiry_time === false) {
 					return null;
 				}