Drop PrestaShop <9 backports, require PHP 8.1+

[nenes25]

Summary

Closes #337.

The 3.x line only targets PrestaShop 9, so this drops the compatibility code paths kept around for older PS/PHP versions, and refreshes tooling accordingly.

• Bump composer.json PHP requirement to >=8.1
• Bump prestashop/php-dev-tools to ^5, pin friendsofphp/php-cs-fixer ^3.2 directly (v5 no longer pulls it in transitively) and symfony/process to ^6.4, matching PrestaShop 9's own Symfony version — this also fixes 2 open Dependabot alerts on symfony/process (moderate + high severity), previously locked at v5.4.35
• Regenerated composer.lock, re-ran PHP-CS-Fixer with the updated toolchain (new default ruleset: comment spacing, blank line after <?php, global-namespace FQCN over use imports for PrestaShop core classes)
• Remove override/controllers/front/OrderController.php: its only logic was gated behind version_compare(_PS_VERSION_, '8.2.1', '<') (always false on PS9) and called a hook method already removed in [enhancement]: Remove AuthController override #335. Verified against the real PS9 core (classes/checkout/CheckoutPersonalInformationStep.php) that the modern checkout flow already fires actionSubmitAccountBefore natively for account creation during checkout, so nothing is lost
• Bump ps_versions_compliancy min from 1.7.0.0 to 9.0.0
• Drop the PHP 1.7.8.11 PHPStan config/ignore rule (RegistrationController always exists on PS9+) and the now-redundant phpstan-8.2.3.neon / phpstan-latest.neon (identical to the base config, the script already falls back to it when no version-specific file exists)
• CI: syntax check and PHPStan matrix now run against PHP 8.1 through 8.5 only (dropped 7.2/7.3/7.4); PHPStan matrix targets prestashop/prestashop:9.1.4-8.x images instead of 1.7.8.11 / 8.2.3

Historical upgrade/*.php scripts are left behaviorally untouched (only whitespace changes from the CS-Fixer pass) since they document past upgrades for existing installs.

Test plan

• composer install succeeds on PHP 8.1
• CI green (syntax check 8.1-8.5, PHP-CS-Fixer, PHPStan 8.1-8.5)
• Install/upgrade module on a PS 9.1.4 test instance
• Guest checkout account creation still validates captcha (via native actionSubmitAccountBefore, no more OrderController override)