Output stderr when in debug mode

[convenient]

Some parts of the build process may output to stderr without failing the pipeline.

For example static content deploy does it here https://github.com/magento/magento2/blob/2.4-develop/app/code/Magento/Deploy/Service/DeployPackage.php#L144-L147

If you have an issue that you want to debug and you set MIN_LOGGING_LEVEL: debug then I think it is unexpected that this data would not be present

Manual testing scenarios

The most obvious part of a deployment process that outputs to stderr would be the di compilation, when you do a deployment with this in place you should get more output there.

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