Read postgres version from configuration

[JackLewis-digirati]

As part of work on #716 It was discovered that generating a guid without an extension in npgsql requires the postgres version to be set to at least 13, whereas previously it wasn't set at all. In this work, it was decided it was ok to set the database version in code, but in the future it would pull this in from configuration instead and this ticket is to track this.