As part of enhancing problem checks and admin notices, we standardized the location of the problem check messages' locale keys. However, we overlooked this one in the plugin, resulting in a missing translation on the dashboard.
This PR puts the locale key in the place expected by the admin notice system.
We're promoting problem checks to first class citizens in core. This migrates the problem check to the new API.
In the process of adding tests for this check, I discovered what seems like a mistake that likely means this check never worked until now. (See inline comment.)