Ensure Symfony/mbstring-polyfill version 1.31 or higher

[ellnix]

Fixes #375

Please let me know if I should include further details, this is my first time contributing to anything in the Laravel ecosystem 😄

[crynobone]

But Laravel Framework itself requires mbstring extension

[ellnix]

But Laravel Framework itself requires mbstring extension

There are changes to the mbstring extension on PHP 8.4 that the polyfill backports. Besides, this polyfill is already a dependency of this project through Symfony/console.