chore: add notice about planned harbor deprecations

[shreddedbacon]

General Checklist

• Affected Issues have been mentioned in the Closing issues section
• Documentation has been written/updated
• PR title is ready for inclusion in changelog

Database Migrations

• If your PR contains a database migation, it MUST be the latest in date order alphabetically

Just adding some notices about planned old Harbor version deprecation. The code in the API is already basically abandoned and likely doesn't work reliably for anyone using Harbor 2.2.x+. And retaining the code base in the remote-controller means retaining a forked client library, for which the newer library doesn't require a fork.

This would allow us to proceed with cleaning up old code in a future release, here are some existing PRs that could be merged then.

• chore: remove remaining harbor integration from core #3259
• Feature/expunge harbor trivy from core #3086
• refactor: remove support for harbor 2.1.x and older remote-controller#242
• any others that might be required to remove support from the API

[tobybellwood]

happy with these, flagging them for people's attention.