Upgrade to PHP 8.5 / Symfony 7.4 and modernize CI, PHPUnit and composer settings

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Motivation

• Bring the package up to modern PHP/Symfony versions and developer tooling by requiring PHP 8.5 and Symfony 7.4-level components.
• Modernize test tooling and CI so tests run on GitHub Actions with stable and lowest dependency matrices and use a current PHPUnit schema.
• Improve composer metadata and autoloading for clearer packaging and developer workflows.

Description

• Updated composer.json to require php:^8.5, Symfony ^7.4 components, doctrine/orm:^3.3, twig:^3.0, and bumped phpunit/phpunit to ^12.0, plus added autoload-dev and packaging config tweaks.
• Added a GitHub Actions workflow at .github/workflows/phpunit.yml that validates composer, caches dependencies, installs stable/lowest deps and runs vendor/bin/phpunit -c phpunit.xml.dist on PHP 8.5.
• Modernized phpunit.xml.dist to the PHPUnit 12 schema, enabled cache directory, adjusted coverage configuration, and set bootstrap to Tests/bootstrap.php.
• Simplified .travis.yml to act as legacy CI with PHP 8.5 and a simple vendor/bin/phpunit -c phpunit.xml.dist script, and updated README.md and UPGRADE.md with new requirements and migration notes.

Testing

• No automated test runs are included in this PR itself; CI is configured to run PHPUnit via GitHub Actions using vendor/bin/phpunit -c phpunit.xml.dist for both stable and lowest dependency modes on PHP 8.5.
• The legacy Travis job was adjusted to run the same PHPUnit command for backward compatibility but no Travis run results are included here.

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Codex Task