Throw exception in getManagerForClass when multiple entity managers manage the same entity

This came to me while reading through https://github.com/symfony/symfony-docs/issues/9878. In a few of those cases, people have the same entity managed by multiple entity managers. While I'm not sure whether that constitutes a valid use-case, the manager registry is equipped to handle this by allowing to specify en entity manager to use when calling `AbstractManagerRegistry::getRepository`.

However, `AbstractManagerRegistry::getManagerForClass` handles those cases wrong IMO: it simply returns the first entity manager that manages the entity in question. In cases like the above, this can lead to undesired behaviour as we're ignoring the fact that we can't reliably say which entity manager is responsible for.a given class.

Assuming the use-case of having the same entity managed by multiple entity managers is valid, we should throw an exception in `getManagerForClass` if we realise that the same class is managed by more than a single entity manager.