discord-{ptb,canary}: update to latest versions and fixup updateScript

This commit:

- Moves the update script into the dir, out of the maintainers dir. This makes
  it more discoverable in general. It can also be invoked from anywhere to write
  to default.nix
- Swaps it to use the standardized `passthru.updateScript`. This means that
  eventually bots like `nixpkgs-update` will be able to handle it.
- Runs the script to upgrade to the latest versions

See https://nixos.org/nixpkgs/manual/#ssec-stdenv-attributes