Ah yes, welcome to the bleeding edge! ArcGIS 10.6 is here in prerelease form. If you have current maintenance, then you have access to the software from my.esri.com.
Make sure to read the release notes. You will find that for ArcGIS Enterprise, upgrading to Prerelease from earlier versions is not supported. You are required to do a new installation.
I also found in the What’s New doc that at 10.6 you can now use the PostGIS geography type to store spatial data in PostgreSQL databases and geodatabases in PostgreSQL. Also PostgreSQL 9.3 is not supported, so upgrade to 9.4.8 or 9.5.3!