docs: require PostgreSQL 7.4 or later

Author: kenjis

user_guide_src/source/intro/requirements.rst

@@ -50,7 +50,7 @@ A database is required for most web application programming.
 Currently supported databases are:
 
   - MySQL via the ``MySQLi`` driver (version 5.1 and above only)
-  - PostgreSQL via the ``Postgre`` driver
+  - PostgreSQL via the ``Postgre`` driver (version 7.4 and above only)
   - SQLite3 via the ``SQLite3`` driver
   - Microsoft SQL Server via the ``SQLSRV`` driver (version 2005 and above only)
   - Oracle Database via the ``OCI8`` driver (version 12.1 and above only)