fix(composer): make composer PHP files v8.1 compatible

[M-arcus]

This pull request includes a small change to the composer/MagoPlugin.php file. The change removes the readonly keyword from the MagoPlugin class definition and adjusts the PACKAGE_NAME constant to include a docblock for type annotation.

• composer/MagoPlugin.php: Removed the readonly keyword from the MagoPlugin class and added a docblock for the PACKAGE_NAME constant.

The following rector config was used:

<?php

declare(strict_types=1);

use Rector\Config\RectorConfig;

return RectorConfig::configure()
    ->withPaths([__DIR__ . '/composer'])
    ->withBootstrapFiles([__DIR__ . '/vendor/autoload.php'])
    ->withDowngradeSets(php81: true);

[azjezz]

@veewee beat you to it 😅 thank you