[Doctrine] Cleaning up the config code samples

[ThomasLandauer]

Page: https://symfony.com/doc/5.4/doctrine/multiple_entity_managers.html

Things I've changed:

1. Moved defaultConnection downwards (YAML & PHP). Setting the default before defining any connection looked awkward.
2. Removed driver, server_version, charset. As I see it on https://symfony.com/doc/current/doctrine.html#configuring-the-database, the recommended way is to set this inside .env. Besides, it distracts from the important points here.
3. Renamed DATABASE_CUSTOMER_URL to CUSTOMER_DATABASE_URL - in accordance with the way the dependency injection works: EntityManagerInterface $customerEntityManager.
   Plus renaming emDefault to defaultEntityManager in PHP config.

[javiereguiluz]

I like this change. The result is much more clear. Thanks Thomas!