[Security] Missing return type in "UserInterface"

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | https://github.com/symfony/symfony/issues/13037
| License       | MIT
| Doc PR        | 
@Turneliusz the current code supports returning a mix of strings and RoleInterface instances. This is precisely why @fabpot said that your proposal does not describe the code expectations