Motivation for this change

about 6 months ago, upstream added the ability to load these variables from a file directly, via ADMIN_USERNAME_FILE and ADMIN_PASSWORD_FILE. This feature is included in the version in nixpkgs, 2.0.23.

Therefore, we don't need the adminCredentialsFile option as it's now redundant. It seems like behavior isn't well defined if both the direct and as-file variables are set, so I've added some janky logic to not set a default if the other is set (though I've tested all 3 possible input states and it seems to work as expected).

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.