fix: move custom API endpoint warning message to stderr

[snykerjames]

• Ready for review
• Follows CONTRIBUTING rules
• Reviewed by Snyk internal team

What does this PR do?

Changes the output stream for the "Using a custom API endpoint..." warning message from stdout to stderr.

Where should the reviewer start?

src/lib/config.ts: line 33

How should this be manually tested?

test with snyk-to-html using

$ snyk test --json | snyk-to-html

Any background context you want to provide?

This message currently outputs to stdout, which when running snyk with --json, breaks tools like snyk-to-html that require valid json input

What are the relevant tickets?

N/A

Screenshots

Before change:

After change:

Additional questions

I didn't find an existing test for this, so would be happy to add one with your guidance.

[CLAassistant]

All committers have signed the CLA.

[gitphill]

Thanks for the contribution!

[snyksec]

🎉 This PR is included in version 1.377.2 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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