doc: 20.09 release notes: nixos-YY.MM branches no longer in nixos-channels repo

[chkno]

Since NixOS/nixos-channel-scripts@7c442a2
for #99257

Motivation for this change

Tell folks that were waiting for the nixos-20.09 branch to appear in the nixpkgs-channels repo as the signal that 20.09 is ready that this signal will never come and they need to quit looking there.

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.

[zowoq]

editorconfig failures will be fixed by #103535.

[Ma27]

Thanks! ALso ported to stable as 3090c65.