If someone uses `utf8` in MySQL 8.0, they will get a warning suggesting they should use `utf8mb4`, because `utf8` will be deprecated.

Redefining `utf8` to mean 4-byte would break the upgrade since existing tables would not be able to join against newly created tables.

This is discussed here: https://mysqlserverteam.com/sushi-beer-an-introduction-of-ut...