Change experimental Gradle parser log event to DEBUG

What does this MR do?

The logging level for this event gave a customer the impression that the scanner was working incorrectly when it was working fine. This merge request tunes the level for the relevant log event, so that it's only output in debug mode. The thread where that conversation happened is here.

What are the relevant issue numbers?

Experimental Gradle Parser Throws Recoverable E... (gitlab-org/gitlab#446145 - closed) • Philip Cunningham • 16.10 • On track

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
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• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer