[VL] set fallback reason as WARN instead of INFO

[FelixYBW]

Fallback reason is important info and should be a WARN.

INFO printed too much lines so I usually set to WARN log level.

@zhli1142015 @Yohahaha @ulysses-you @jackylee-ch @kecookier @WangGuangxin
Your thought?

[Yohahaha]

👍 spark.gluten.sql.validation.logLevel was set to WARN by default in our product.

[FelixYBW]

We will set it to WARN since no object.

[kecookier]

I agree with your suggestion. In my situation, we will also collect fallback information and send it to Kafka for further analysis. Thank you for your input!

[FelixYBW]

#6676 set default to WARN.

I didn't note we already have a config to control this.