[Task]: The "Manifest V3 compatibility" warning during submission should really only apply to extensions

[willdurand]

Description

1. Open a valid MV3 theme
2. Observe the "Manifest V3 compatibility" warning

Expected

No warning since (1) the warning message is about extensions and (2) there shouldn't be any warning for MV3 non-extension add-ons since nothing has changed for these add-on types.

Acceptance Criteria

The "Manifest V3 compatibility" warning isn't shown for non-extension add-ons.

Checks

• If the issue is ready to work on, I have removed the "needs:info" label.
• If I have identified that the work is specific to a repository, I have removed "repository:addons-server" or "repository:addons-frontend"

┆Issue is synchronized with this Jira Task