Set current_version attribute to postgresql datastore

Fixing the migration upgrade cli use.

Related to swh-core#4305 (closed)

Test Plan

Trying it out on staging does fix the version cli (so the other ones as well).

swhstorage@storage1:~$ swh db --config-file config.yml version storage
WARNING the database does not have a dbmodule table.
module: storage
flavor: default
current code version: 182
version: 183

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Migrated from D7953 (view on Phabricator)