Fix deprecation with next version of League\OAuth2

[jderusse]

This PR fixes deprecations triggered by the SymfonyConnect:

Method "League\OAuth2\Client\Provider\AbstractProvider::getBaseAuthorizationUrl()" might add "string" as a native return type declaration in the future. Do the same in child class "Qdequippe\OAuth2\Client\Provider\SymfonyConnect" now to avoid errors or add an explicit @return annotation to suppress this message.

The composer's file of this project does not contain a minimal version for PHP, I assume, that all versions are supported.
But their is the league/oauth2-client: ^2.0 constraint that implicitly requires the minimal version of league/oauth2-client 2.0 which is PHP >=5.6.0.

The scalar return type was introduced in PHP 7.0, That's why, I couldn't add a proper return type in this PR, and I had to use a doc bloc + annotation

[qdequippe]

Thanks @jderusse 👍