ProgressBar should not force to write on stderror

[coExp]

Using the Symfony ProgressBar, I discovered that it's writen on stderror, and not on stdout.
In my case, I need to catch messages from sterror.

[symfony-bot]

Thanks for your pull request! We love contributions.

However, this repository is what we call a "subtree split": a read-only copy of one directory of the main Symfony repository. It is used by Composer to allow developers to depend on specific Symfony components.

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PS: if you haven't already, please add tests, and beware that bug fixes should be submitted on the lowest maintained branch where they apply; only features should be submitted against the master branch.