nixos/nagios: replace ssmtp package use with msmtp

The ssmtp program is not maintained and is being removed.

GitHub: see https://github.com/NixOS/nixpkgs/issues/105710

nixos/modules/services/monitoring/nagios.nix

@@ -102,8 +102,8 @@ in
 
       plugins = mkOption {
         type = types.listOf types.package;
-        default = with pkgs; [ monitoring-plugins ssmtp mailutils ];
-        defaultText = literalExpression "[pkgs.monitoring-plugins pkgs.ssmtp pkgs.mailutils]";
+        default = with pkgs; [ monitoring-plugins msmtp mailutils ];
+        defaultText = literalExpression "[pkgs.monitoring-plugins pkgs.msmtp pkgs.mailutils]";
         description = "
           Packages to be added to the Nagios <envar>PATH</envar>.
           Typically used to add plugins, but can be anything.