networkmanager-fortisslvpn: Mark as vulnerable

https://blogs.gnome.org/mcatanzaro/2026/09/15/privilege-escalation-vulnerabilities-in-networkmanager-plugins/
https://gitlab.gnome.org/GNOME/NetworkManager-fortisslvpn/-/work_items/80

Also stop recommending it in NM NixOS module docs.

nixos/modules/services/networking/networkmanager.nix

@@ -241,7 +241,6 @@ in
         default = [ ];
         example = literalExpression ''
           with pkgs; [
-            networkmanager-fortisslvpn
             networkmanager-iodine
             networkmanager-libreswan
             networkmanager-l2tp

pkgs/by-name/ne/networkmanager-fortisslvpn/package.nix

@@ -90,6 +90,9 @@ stdenv.mkDerivation rec {
   meta = {
     description = "NetworkManager’s FortiSSL plugin";
     inherit (networkmanager.meta) maintainers teams platforms;
+    knownVulnerabilities = [
+      "CVE-2026-91839"
+    ];
     license = lib.licenses.gpl2Plus;
   };
 }