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

maintainers/scripts/update-discord

@@ -1,36 +0,0 @@
-#!/usr/bin/env bash
-# script to generate `pkgs/networking/instant-messengers/discord/default.nix`
-
-set -e
-exec >${1:?usage: $0 <output-file>}
-
-cat <<EOF
-{ branch ? "stable", pkgs }:
-# Generated by /maintainers/scripts/update-discord
-let
-  inherit (pkgs) callPackage fetchurl;
-in {
-EOF
-
-for branch in "" ptb canary; do
-    url=$(curl -sI "https://discordapp.com/api/download${branch:+/}${branch}?platform=linux&format=tar.gz" | grep -oP 'location: \K\S+')
-    version=${url##https://dl*.discordapp.net/apps/linux/}
-    version=${version%%/*.tar.gz}
-    echo "  ${branch:-stable} = callPackage ./base.nix rec {"
-    echo "    pname = \"discord${branch:+-}${branch}\";"
-    case $branch in
-        "") suffix="" ;;
-        ptb) suffix="PTB" ;;
-        canary) suffix="Canary" ;;
-    esac
-    echo "    binaryName = \"Discord${suffix}\";"
-    echo "    desktopName = \"Discord${suffix:+ }${suffix}\";"
-    echo "    version = \"${version}\";"
-    echo "    src = fetchurl {"
-    echo "      url = \"${url//${version}/\$\{version\}}\";"
-    echo "      sha256 = \"$(nix-prefetch-url "$url")\";"
-    echo "    };"
-    echo "  };"
-done
-
-echo "}.\${branch}"