[incompatible] change log output dest to stderr

[Songmu]

Logs should generally be sent to stderr, and stdout should be used for pipeline processing.
Specifically, we make this incompatibility change first because we want to output updated files to stdout to coordinate with other processes.

[coveralls]

Pull Request Test Coverage Report for Build 6597857246

• 9 of 9 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.0%) to 31.312%

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Totals
Change from base Build 6597726757:	1.0%
Covered Lines:	222
Relevant Lines:	709

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💛 - Coveralls