Align schema handling with prooph event store

[codeliner]

The document store performs a mb_strtolower when extracting the schema, see here: https://github.com/event-engine/php-postgres-document-store/blob/master/src/PostgresDocumentStore.php#L774

Prooph's event store uses the schema as-is. That said, when using the MultiModelStore this can lead to problems and confusion.

We need to address the issue.

Options

1. Remove mb_strtolower (breaking change, but it's relatively new behavior, so maybe not a big deal)
2. Modify ProophEventStore wrapper and perform a mb_strtolower before passing the schema name to prooph's event store

[codeliner]

ping @sandrokeil @heiglandreas