Resolve Composer dependencies for PHP 8.5 #100

Dependabot's Composer update (#90) downgrades 11 Symfony packages from 8.1.x to 8.0.x, for example symfony/string 8.1.7 → 8.0.15. composer.json requires "php": "^8.4" and doesn't set a platform PHP version, so Dependabot resolves for PHP 8.4.0, and Symfony 8.1 needs 8.4.1 or newer. The servers and CI run PHP 8.5.

• Set config.platform.php to 8.5 in composer.json, so Composer and Dependabot resolve for the PHP version we run.
• Refresh composer.lock's hash without changing any package.
• After merging, @dependabot recreate Bump the composer-minor-and-patch group across 1 directory with 2 updates #90 and check it no longer downgrades anything.