Apparently libpq version 9.1.9 is no longer supported.
Let's use version 9.6 to keep maximum compatibility and align
the version with MacOS.
Task-number: QTBUG-79555
Change-Id: Iea98a1eb821f103e3a610ed602746407bfe5ea97
Reviewed-by: Tony Sarajärvi <tony.sarajarvi@qt.io>