Update GitLab Duo Chat icon

What does this MR do and why?

Replaces the generic tanuki-ai icon with duo-chat or duo-chat-off

Closes #473396 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After

Before	After

How to set up and validate locally

1. From the rails console enable feature flag:

Feature.enable(:duo_chat_disabled_button)

2. Log in as admin to your local GDK instance.
3. Disable Duo Chat for a project from the console:

project = Project.find_by(name: "Gitlab Smoke Tests")
project.update!(duo_features_enabled: false)

4. Visit the project - the button should be visible.
5. Inspect the button and you should see a chat_disabled_reason attribute with a project value.
6. Visit a project with enabled Duo Chat and inspect the button. It should not have the chat_disabled_reason attribute.
7. With the flag disabled, for project with disabled Duo Chat, the button should not be visible.