A while ago we added the channel branches (nixos-* etc.) to the nixpkgs repository, so nixpkgs-channels is no longer necessary. However it still exists, which confuses people. I propose to archive it. Thus existing references will still work but we won't add new channels to it.

Concretely:

• In the channel mirroring script, stop mirroring NixOS >= 20.09. (Also delete the nixos-20.09 branches in nixpkgs-channels. It's not released yet so we can do that.)
• Once we stop updating NixOS < 20.09, archive the nixpkgs-channel repo.