Notation Verify should warnings output to Stderr

[byronchien]

What is the areas you experience the issue in?

Notation CLI

What is not working as expected?

notation outputs warnings in some cases (such as when tag references are resolved), and these warnings are written to stdout.

What did you expect to happen?

I expect any warnings to be written to stderr instead of stdout.

How can we reproduce it?

Can reproduce this behavior by running notation sign or verify with a tag reference, ex: notation verify localhost:5000/net-monitor:v1

Describe your environment

go version 1.19.2, darwin/amd64

What is the version of your Notation CLI or Notation Library?

v1.0.0-rc.1+unreleased