nixos/discourse: Assert deployed PostgreSQL version

Assert that the PostgreSQL version being deployed is the one used
upstream. Allow the user to override this assertion, since it's not
always possible or preferable to use the recommended one.

nixos/tests/discourse.nix

@@ -51,6 +51,8 @@ import ./make-test-python.nix (
 
         environment.systemPackages = [ pkgs.jq ];
 
+        services.postgresql.package = pkgs.postgresql_13;
+
         services.discourse = {
           enable = true;
           inherit admin;