update postgresql requirements

[skip ci]

doc/install/requirements.md

@@ -121,8 +121,8 @@ Existing users using GitLab with MySQL/MariaDB are advised to
 
 ### PostgreSQL Requirements
 
-As of GitLab 9.3, PostgreSQL 9.2 or newer is required, and earlier versions are
-not supported. We highly recommend users to use at least PostgreSQL 9.6 as this
+As of GitLab 10.0, PostgreSQL 9.6 or newer is required, and earlier versions are
+not supported. We highly recommend users to use PostgreSQL 9.6 as this
 is the PostgreSQL version used for development and testing.
 
 Users using PostgreSQL must ensure the `pg_trgm` extension is loaded into every