postgresql: branch 9.3 is not maintained anymore

See https://www.postgresql.org/support/versioning/.

pkgs/servers/sql/postgresql/default.nix

@@ -106,6 +106,8 @@ let
       license     = licenses.postgresql;
       maintainers = with maintainers; [ ocharles thoughtpolice ];
       platforms   = platforms.unix;
+      knownVulnerabilities = optional (!atLeast "9.4")
+        "PostgreSQL versions older than 9.4 are not maintained anymore!";
     };
   });