Merge branch '6.4' into 7.2

* 6.4:
  add translations for the Twig constraint
  drop the Date header using the Postmark API transport
  [VarExporter] Fix: Use correct closure call for property-specific logic in $notByRef
  Unnecessary cast, return, semicolon and comma
  [Notifier] [Discord] Fix value limits
  Fix ServiceMethodsSubscriberTrait for nullable service

Author: fabpot

Tests/Extension/Core/DataTransformer/DateTimeToLocalizedStringTransformerTest.php

@@ -238,7 +238,7 @@ public function testReverseTransformFromDifferentLocale()
     {
         if (version_compare(Intl::getIcuVersion(), '71.1', '>')) {
             $this->markTestSkipped('ICU version 71.1 or lower is required.');
-        };
+        }
 
         \Locale::setDefault('en_US');