[config] Remove settings of unused DB drivers
[config] Make DB driver settable by environment
[config] Update MySQL settings

This change defines a ‘main’ connection that is not implying any specific database driver. It also removes the fallback values of the different settings.
[database] Remove built-in migrations

These are not needed for this project.
[config] Use `env()` helper in DB config file
[config] Add settings for `migration` connection

It is good practice to restrict the rights of the main database user to just what’s required to perform normal operations.
In this case, an additional user with more rights is needed to execute the database migrations.

This change adds optional settings allowing to use a migration-specific database connection.

If the related environment variables are not defined, the credentials fall back on those of the main connection.