services: postgresql: upgrade to 14

Followed https://nixos.org/manual/nixos/stable/#module-postgresql and
everything seems to have gone just fine.

services/postgresql.nix

@@ -5,6 +5,6 @@
 }: {
   # set postgresql version so we don't get any bad surprise
   config.services.postgresql = {
-    package = pkgs.postgresql_13;
+    package = pkgs.postgresql_14;
   };
 }