QPSQL: Fix check for minimum supported PostgreSQL version

According to Qt documentation http://doc.qt.io/qt-5/sql-driver.html#qpsql
minimum supported version of PostgreSQL is 7.3

Change-Id: I30cffaddc29fd56b534bfd259cc235ea1204a21f
Reviewed-by: Andy Shaw <andy.shaw@qt.io>

src/plugins/sqldrivers/psql/qsql_psql.cpp

@@ -843,7 +843,7 @@ QPSQLDriver::Protocol QPSQLDriverPrivate::getPSQLVersion()
     if (serverVersion == QPSQLDriver::VersionUnknown)
         serverVersion = QPSQLDriver::Version6;
 
-    if (serverVersion < QPSQLDriver::Version7_1) {
+    if (serverVersion < QPSQLDriver::Version7_3) {
         qWarning("This version of PostgreSQL is not supported and may not work.");
     }