Tighten GitHub Actions / Drop Laravel 11 Support

[ultrono]

Add three new steps to the GitHub Actions workflow:

• Composer validation — checks composer.json is well-formed and strict
• PHP lint — runs a syntax check across all files in src/
• Dependency audit — flags any installed packages with known vulnerabilities

These are cheap, fast checks that catch real problems early:

• A malformed composer.json or syntax error will now fail the build immediately, before wasting time installing dependencies or running the full test suite
• The security audit means vulnerabilities in dependencies are caught automatically on every push, rather than discovered later in production

All three steps use built-in PHP/Composer tooling — no new dependencies required.

The validate and lint steps run after PHP is set up via shivammathur/setup-php, so they use the correct matrix PHP version rather than the runner default.

Due to composer audit output and the fact Laravel 11 is now officially EOL (see https://laravel.com/docs/13.x/releases#support-policy), this PR also drops Laravel 11 support.