Config overhaul: lowercase for `DatabaseDriver`

Parent issue: https://github.com/torrust/torrust-tracker/issues/401

We are normalizing all enum variants to lowercase. `DatabaseDriver` is not normalized yet.

```rust
pub enum DatabaseDriver {
    /// The Sqlite3 database driver.
    Sqlite3,
    /// The `MySQL` database driver.
    MySQL,
}
```