Use WARN logging for agent shutdown message

[tegefaulkes]

Specification

The change made in #157 was not done fully according to our standards. We should use the existing logging system for this message. So the following changes need to be made.

1. Remove the agent stopping message from the agent start command.
2. Covert the agent stopping and agent stopped logger messages in polykey to the WARN level so they are visible with default verbosity in the CLI.

This will allow us to keep the shutting down message while keeping to the logging system.

Additional context

Related : #157

Tasks

1. Remove the agent stopping message from the agent start command.
2. Covert the agent stopping and agent stopped logger messages in polykey to the WARN level so they are visible with default verbosity in the CLI.

[linear]

ENG-399 Use WARN logging for agent shutdown message