fix: use project_dir if available (Symfony 3+)

[alanpoulain]

kernel.root_dir is deprecated since Symfony 4 and removed since Symfony 5.

kernel.project_dir has been introduced in Symfony 3.

See also https://symfony.com/blog/new-in-symfony-3-3-a-simpler-way-to-get-the-project-root-directory.

In this PR, I tried to be compatible with Symfony 2 and Symfony 3+ by using a phobetor_rabbitmq_supervisor.project_dir parameter set in the extension.

The phobetor_rabbitmq_supervisor.project_dir will correspond to the path where the composer.json file is, in all the cases.

The defaults paths for the supervisor configuration will then be at the same place as the composer.json file.
If you prefer to have the defaults paths in the var folder by default, please tell me.

For information:
With Symfony 2 (not flex), kernel.root_dir is used and corresponds to the app directory.
With Symfony 3+ (not flex), kernel.root_dir is also used and corresponds to the app directory.
With Symfony 3+ (flex), kernel.root_dir is used and corresponds to the src directory.
With Symfony 6 (flex), kernel.project_dir is used and corresponds to the directory where the composer.json file is.

[Kitton]

@Phobetor any chance for this PR to get merged and released? Also, #69 is related