feat: Only stream command output to stdout

[mildaniel]

Since sam list is a command that can be used programmatically with the json output format, we should reserve the stdout stream to be exclusively the output of the command, similar to how sam local invoke works today.

This PR changes the warning message to use stderr instead.

Mandatory Checklist

PRs will only be reviewed after checklist is complete

• Add input/output type hints to new functions/methods
• Write design document if needed (Do I need to write a design document?)
• Write/update unit tests
• Write/update integration tests
• Write/update functional tests if needed
• make pr passes
• make update-reproducible-reqs if dependencies were changed
• Write documentation

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.