Use logger.warn for promo banner to enable silencing

[david0]

Problem: The promo banner is shown on every run of a task when using gradle (#11211). There is no way to disable it.

Solution: This will allow to silence this message on by running gradlew -q ...

(More could be done on that topic like adding a config switch and/or showing the banner just once, but I decided to keep it simple)

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