[#2909] Change session validation log messages from INFO to DEBUG

[lprimak]

fixes #2909

Summary of Changes

• Downgrades routine session validation logging ("Validating all active sessions..." and "Finished session validation.") from INFO to DEBUG in AbstractValidatingSessionManager.
• Reduces log noise during periodic session cleanup cycles while preserving diagnostic tracing when needed.

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[steinarb]

LGTM!