nixos/tests: remove unnecessary stateVersion assignments

nixos/tests/containers-imperative.nix

···
           modules = lib.singleton {
             nixpkgs = { inherit (config.nixpkgs) localSystem; };
 
-            containers.foo.config = {
-              system.stateVersion = "18.03";
-            };
+            containers.foo.config = {};
           };
 
           # The system is inherited from the host above.

nixos/tests/nextcloud/basic.nix

···
         "d /var/lib/nextcloud-data 0750 nextcloud nginx - -"
       ];
 
-      system.stateVersion = "22.11"; # stateVersion >=21.11 to make sure that we use OpenSSL3
-
       services.nextcloud = {
         enable = true;
         datadir = "/var/lib/nextcloud-data";