PostgreSQL: don't use `regconfig` and `GENERATED` column anymore /!\ pgsql schema needs to be updated /!\ /!\ Minimal PostgreSQL required version is back to 9.5 /!\ `regconfig` is using system table, and `pg_upgrade` can't handle that, causing trouble when moving to a new major version of PostgreSQL. To work around this, the `data_fts_cfg` column type in `items` has been changed from `regconfig` to `text`. GENERATED column can't be used with type casting to `regconfig`, so the data_fts column is now generated with a trigger. As a result, the minimal requirement of PostgreSQL 12 is not necessary anymore.