Minor symfony 6.1 deprecation

[ehymel]

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After upgrading to symfony 6.1, I'm getting this new deprecation:

Method "Symfony\Component\Config\Definition\ConfigurationInterface::getConfigTreeBuilder()" might add "TreeBuilder" as a native return type declaration in the future. Do the same in implementation "SecIT\ImapBundle\DependencyInjection\Configuration" now to avoid errors or add an explicit @return annotation to suppress this message.

Seems like an easy fix...