nixos/virtualisation/linode-image: Migrate to new openssh options.

nixos/modules/virtualisation/linode-config.nix

···
   services.openssh = {
     enable = true;
 
-    permitRootLogin = "prohibit-password";
-    passwordAuthentication = mkDefault false;
+    settings.PermitRootLogin = "prohibit-password";
+    settings.PasswordAuthentication = mkDefault false;
   };
 
   networking = {