Fix EZP-22925: Database setting injection is broken

This PR resolves issue https://jira.ez.no/browse/EZP-22925

The injection of RDBMS handler implementation into Legacy Stack was broken since DoctrineBundle integration. The problem appears when changing RDBMS after SetupWizard.

This fixes injection and adds `RuntimeException` when driver can't be mapped to LS handler.

Manually tested.