Upgrade Mailgun Webhooks Bundle to PHP 8.5, Symfony 7.4 and Symfony Mailer

[azine]

What changed

• upgrades the bundle to PHP 8.5, Symfony 7.4, Twig 3 and Doctrine ORM 3;
• declares the required curl, json and mailparse PHP extensions;
• migrates notification delivery from Swiftmailer to Symfony Mailer/Mime while preserving the existing service API;
• modernizes Doctrine persistence namespaces, configuration trees and PHPUnit tests;
• replaces the broken recursive subprocess retry command with bounded in-process retry behavior;
• updates GitHub Actions to Node 24-compatible actions and tests stable plus lowest dependency sets.

Validation

The branch is green on PHP 8.5 for both stable and lowest dependency matrices. It still requires the final application-level integration run together with the Azine.Me upgrade before release.