Update database driver in Tracker configuration

Relates to: https://github.com/torrust/torrust-tracker/pull/953

After merging this [PR](https://github.com/torrust/torrust-tracker/pull/953) the tracker will use lowercase values for the database driver. We have to update the tracker configuration.