[php-src] Issue #8351: explicit `__construct` call for \DateTimeImmutable object would break immutable

Issue: https://github.com/php/php-src/issues/8351 Author: sasezaki ### Description The following code: ```php <?php $date = new \DateTimeImmutable('1970-01-01 00:00:00.000000'); $date->__construct('2000-01-01 00:00:00.000000'); var_dump($date); ``` Resulted in this output: ``` object(DateTimeImmutable)#1 (3) { ["date"]=> string(26) "2000-01-01 00:00:00.000000" ["timezone_type"]=> int(3) ["timezone"]=> string(16) "Europe/Amsterdam" } ``` https://3v4l.org/hnnIE But I expected this output instead: ``` Fatal error: Uncaught Error: Cannot modify readonly property DateTimeImmutable::$date ``` I did learn this issue when reading this slide. https://speakerdeck.com/twada/growing-reliable-code-phperkaigi-2022?slide=84 According to the author of this slide, twada, he noticed this problem himself. (after reading https://www.simonholywell.com/post/2017/03/php-and-immutability/ ) I propose to give readonly to each of the properties of \DateTimeImmutable. ### PHP Version PHP 8.1.4 ### Operating System _No response_