random: Fix unknown `mt_srand()` compatibility for unknown modes (#13544)

PHP 8.1 and below interpreted unknown modes as `MT_RAND_MT19937`, but PHP 8.2+
interprets them as `MT_RAND_PHP`.

random: Fix unknown `mt_srand()` compatibility for unknown modes (#13544)

PHP 8.1 and below interpreted unknown modes as `MT_RAND_MT19937`, but PHP 8.2+
interprets them as `MT_RAND_PHP`.

Align the behavior with PHP 8.1 and below, because folks should be steered
towards the standard mode.

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