Allow Symfony 6

[alexislefebvre]

When PHP 8.1 will be released and available on CI, and when Symfony 6 will be released:

• remove the dependencies that were added only to allow the unstable versions
• replace ^6.0.0-BETA3 with ^6.0
• restore PHPCR for Symfony 6, it will require Allow Symfony 6 doctrine/phpcr-odm#821 and an update in https://github.com/doctrine/DoctrinePHPCRBundle/blob/master/composer.json → see Restore PHPCR for Symfony 6 #161

[alexislefebvre]

Tests are broken because the calls to self::$container don't work with Symfony 6, it was deprecated then removed:

    /**
     * @var ContainerInterface
     *
     * @deprecated since Symfony 5.3, use static::getContainer() instead
     */
    protected static $container;

We have to see if getContainer() exist in Symfony 4.