armcord,mailspring: mark as insecure (CVE-2023-4863)

See #254798. Upstream has not provided any update for this critical
vulnerability in > 2 weeks. These programs are also likely vulnerable to
many more old vulnerabilities due to using EOL versions of Electron.

pkgs/applications/networking/instant-messengers/armcord/default.nix

@@ -138,5 +138,6 @@ stdenv.mkDerivation rec {
     maintainers = with maintainers; [ ludovicopiero wrmilling ];
     platforms = [ "x86_64-linux" "aarch64-linux" ];
     mainProgram = "armcord";
+    knownVulnerabilities = [ "CVE-2023-4863" ];
   };
 }

pkgs/applications/networking/mailreaders/mailspring/default.nix

@@ -99,5 +99,6 @@ stdenv.mkDerivation rec {
     homepage = "https://getmailspring.com";
     downloadPage = "https://github.com/Foundry376/Mailspring";
     platforms = [ "x86_64-linux" ];
+    knownVulnerabilities = [ "CVE-2023-4863" ];
   };
 }