This got broken up into separate `replication_factor` and
`consistency_mode` settings with Garage 1.x, and due to the the "none"
default kicking in, Garage fails to startup with
> : Error: Either the legacy replication_mode or replication_level and consistency_mode can be set, not both.
if we actually make the migratiom as documented in the migration guide.
Drop this explicit setting, so users can set replication_mode or
replication_factor/consistency_mode, depending on the version they're
using.
8950e22d8c