Werk #15078: Merge the postgres_version check into postgres_instances

So far, the check plugin postgres_version (services "PostgreSQL Version ...") reported versions of PostgreSQL instances. With this werk, we remove this plugin and instead report the version information in the plugin postgres_instances (services "PostgreSQL Instance ..."). This makes the monitoring of PostgreSQL more consistent with the monitoring of other databases such as Oracle.

Users should execute a service re-discovery on affected hosts to remove already discovered PostgreSQL version services.

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