Update `databases` key to use `DB_CONNECTION` from .env

[maskeynihal]

Utilizing env('DB_CONNECTION') enables the package to function seamlessly without requiring any adjustments to the backup.php configuration file. It automatically derives the database connection information from the corresponding environment variable, ensuring flexibility in database selection based on the environment.

[freekmurze]

I think this is a good idea, but it could be considered a breaking change, as there are situations this change will result in a different db being backed up. I'll revisit this when creating a new major version of this package.