Sync intlcal_create_instance() and IntlCalendar::createInstance()

Remove the explicit mention of IntlGregorianCalendar in the latter.
It is a subclass of IntlCalendar, and as such covered if only
IntlCalendar is used as the return type.

ext/intl/calendar/calendar.stub.php

@@ -8,7 +8,7 @@ class IntlCalendar
 
     /**
      * @param IntlTimeZone|DateTimeZone|string|null $timezone
-     * @return IntlCalendar|IntlGregorianCalendar|null
+     * @return IntlCalendar|null
      * @alias intlcal_create_instance
      */
     public static function createInstance($timezone = null, ?string $locale = null) {}

ext/intl/calendar/calendar_arginfo.h

@@ -1,5 +1,5 @@
 /* This is a generated file, edit the .stub.php file instead.
- * Stub hash: ee755d4a500e2d1ba4d589c233b7d09a06b5cce8 */
+ * Stub hash: 18a92d3af801f11e5c3b90e5d272fd98b3515c40 */
 
 ZEND_BEGIN_ARG_INFO_EX(arginfo_class_IntlCalendar___construct, 0, 0, 0)
 ZEND_END_ARG_INFO()