\DateTime::ISO8601 is deprecated use RFC3339 instead

Closes #3474078

src/Plugin/DateTimeEnhancerBase.php

@@ -12,7 +12,7 @@ abstract class DateTimeEnhancerBase extends ResourceFieldEnhancerBase {
    */
   public function defaultConfiguration() {
     return [
-      'dateTimeFormat' => \DateTime::ISO8601,
+      'dateTimeFormat' => \DateTime::RFC3339,
     ];
   }