Assignee widget should check only one assignee at a time in CE

Assignee widget in boards (both classic and refactored versions) should display only one user as an assignee at a time when multiple assignee feature (it's a GitLab Premium feature) is not available.

And in the refactored boards, assignee widget is not aware of the availability of the multiple assignee feature.

❗ Assignee widget is currently behind a feature flag for MR, issue and design pages. Only boards use it by default.