text: Title: Change the log level of `Set Log Level` message from INFO to DEBUG
  Body: When using `Set Log Level`, it logs a message like
  
  > Log level changed from DEBUG to INFO.
  
  using the INFO level. This message can be useful when debugging issues related to logging, but typically it doesn't add much value. As discussed in #4919, it gets especially annoying if want to disable logging while flattening.
  
  For example, a keyword like
  
  ```robotframework
  Example
      [Tags]    robot:flatten
      Log    Message that we want to preserve.
      ${old} =    Set Log Level    NONE
      Log    Message that we want to remove.
      Set Log Level    ${old}
      Log    Another message that we want to preserve.
  ```
  
  will yield following messages:
  
  ```
  15:31:16.892 	INFO 	Message that we want to preserve. 	
  15:31:16.893 	INFO 	Log level changed from NONE to INFO. 	
  15:31:16.893 	INFO 	Another message that we want to preserve.
  ```
  
  The message about changing the level from NONE to INFO is totally useless and distracting. See #4919 for a real world example.