phpfpm: Rename `extraConfig` option to `settings`

[shivaraj-bh]

To conform with the NixOS module’s pool settings option, see https://github.com/NixOS/nixpkgs/blob/0b9a77fcc43a4bb0e23dc17f42d720428bf435fc/nixos/modules/services/web-servers/phpfpm/default.nix#L120-L145

See also: #545 (comment)