Deprecate support for non UserInterface objects

Author: mbabker

src/Authentication/ConnectionRepositoryInterface.php

@@ -27,7 +27,9 @@ public function findAllWithRoles(Topic $topic, array $roles): array;
     public function findTokenForConnection(ConnectionInterface $connection): TokenInterface;
 
     /**
-     * @return string|\Stringable|UserInterface
+     * @return string|\Stringable|UserInterface|null
+     *
+     * @note As of 4.0, the return type will change to `UserInterface|null`.
      */
     public function getUser(ConnectionInterface $connection);
 }