Match nixpkgs support for services.postgresql.extensions

[dbaynard]

In nixpkgs, services.postgresql.extraPlugins has been deprecated. I believe that means this module should support the new attr, although I don't know how to handle the deprecation.

https://github.com/NixOS/nixpkgs/blob/ac24885fb5e9b15729a5fa78f14cfe159e5a86ec/nixos/modules/services/databases/postgresql.nix#L96-L99

    (mkRenamedOptionModule
      [ "services" "postgresql" "extraPlugins" ]
      [ "services" "postgresql" "extensions" ]
    )

Change is needed here, I think?

nix-darwin/modules/services/postgresql/default.nix

Lines 9 to 12 in 349a74c

	postgresql =
	if cfg.extraPlugins == []
	then cfg.package
	else cfg.package.withPackages (_: cfg.extraPlugins);