Fix DateTime exception hierarchy for DateInvalidTimeZoneException (#11970)

`DateInvalidTimeZoneException` is supposed to inherit from `DateException`, but the current stub has `Exception`.

Author: Ayesh

ext/date/php_date.stub.php

@@ -780,7 +780,7 @@ class DateException extends Exception
 /**
  * @strict-properties
  */
-class DateInvalidTimeZoneException extends Exception
+class DateInvalidTimeZoneException extends DateException
 {
 }