ffmpeg_2: mark as insecure

[dotlambda]

Motivation for this change

It's outdated and insecure.
closes #93984 and closes #120371

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

I did not test whether the packages actually still work, just whether they build and run.

Two packages remain:

• stepmania might compile with a newer Ffmpeg if we use a more recent revision (no release made though) or with the vendored Ffmpeg. cc @RubenAstudillo
  Package needs a new maintainer.
• pcsxr should also work if updated to 1.9.95. See https://aur.archlinux.org/packages/pcsxr. I gave it a try but stopped after some complications. cc @rardiol

[Profpatsch]

Does ultrastardx still work with ffmpeg3?

[dotlambda]

Does ultrastardx still work with ffmpeg3?

I was able to build and run it. It played some sounds but I didn't test any functionality.
However, we are also trying to get rid of ffmpeg_3: #120705

[dotlambda]

backport: #121908