nixos/mobilizon: fix integration test by using postgresql_14

nixos/tests/mobilizon.nix

@@ -10,7 +10,7 @@ import ./make-test-python.nix ({ lib, ... }:
     meta.maintainers = with lib.maintainers; [ minijackson erictapen ];
 
     nodes.server =
-      { ... }:
+      { pkgs, ... }:
       {
         services.mobilizon = {
           enable = true;
@@ -25,6 +25,8 @@ import ./make-test-python.nix ({ lib, ... }:
           };
         };
 
+        services.postgresql.package = pkgs.postgresql_14;
+
         security.pki.certificateFiles = [ certs.ca.cert ];
 
         services.nginx.virtualHosts."${mobilizonDomain}" = {