Motivation for this change

The keys.target is used to indicate whether all NixOps keys were
successfully uploaded on an unattended reboot. However this can cause
startup issues e.g. with NixOS containers (see #67265) and can block
boots even though this might not be needed (e.g. with a dovecot2
instance running that doesn't need any of the NixOps keys).

As described in the NixOps manual[1], dependencies to keys should be
defined like this now:

{
  systemd.services.myservice = {
    after = [ "secret-key.service" ];
    wants = [ "secret-key.service" ];
  };
}

However I'd leave the issue open until it's discussed whether or not to
keep keys.target in nixpkgs.

[1] https://nixos.org/nixops/manual/#idm140737322342384

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• 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 nix-review --run "nix-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.

Notify maintainers

cc @