Support manually setting the db version for psql bin

The following discussion from !3291 (merged) should be addressed:

• @twk3 started a discussion: (+1 comment)

  @ibaum If I have a 12.0 that I have upgraded, but skipped the pg-upgrade. Then I took a backup, and wanted to restore that backup to a new 12.0 instance, how do I go about switching that new instance to 9.6?

  (I assume I need to manually create the PG_VERSION file in the right directory, and then run reconfigure, but we should document this somewhere)