[13.x] remove `mt_srand()` deprecated "mode" argument (#60020)

* remove `mt_srand()` deprecated "mode" argument

The "MT_RAND_PHP" mode uses an incorrect Mersenne Twister implementation which was used as the default up till PHP 7.1.0. This mode is only available for backward compatibility.  It has also been deprecated in 8.3.

This commit removes the deprecated mode, and falls back to the default mode of "MT_RAND_MT19937".

https://www.php.net/manual/en/function.mt-srand.php

* fix assertions

with the new mode, the deterministic "random" output has changed, so we need to adjust.