Switching security algorithm from bcrypt to auto breaks existing passwords

**Symfony version(s) affected**: 4.3.0

UPGRADE-4.3 says:
> Configuring encoders using argon2i or bcrypt as algorithm has been deprecated, **use auto instead.**

After changing to auto, all the existing users (with the password starting with "$2y$13$..." can't login anymore. 

Shouldn't `auto` accept also previously stored passwords?
