v3 alpha notice

[kobajagi]

Description

Add notice about state of v3.

Checklist

(For exoscale contributors)

• [x] Changelog updated (under Unreleased block)
• [x] For a new resource or new attributes: test added/updated

[shortcut-integration[bot]]

This pull request has been linked to Shortcut Story #84853: egoscale v3: alpha notice.

[pierre-emmanuelJ]

Thanks, @kobajagi :)