youtube-dl: mark as vulnerable

YouTube-dl has beeun maintained since late 2021, and given the security
issues we've seen with yt-dlp¹ it prudent to make sure nixpkgs moves
away from it.

[1] https://github.com/yt-dlp/yt-dlp/security

pkgs/tools/misc/youtube-dl/default.nix

@@ -92,5 +92,8 @@ buildPythonPackage rec {
     maintainers = with maintainers; [ bluescreen303 fpletz ];
     platforms = with platforms; linux ++ darwin;
     mainProgram = "youtube-dl";
+    knownVulnerabilities = [
+      "youtube-dl is unmaintained, migrate to yt-dlp, if possible"
+    ];
   };
 }