Update backwards compatible settings that work in NixOS but are

deprecated in Nix Darwin to the new standards

nix/system/common/nix.nix

@@ -7,20 +7,25 @@
   nix = {
     settings = {
       experimental-features = "nix-command flakes";
-      auto-optimise-store = true;
       max-jobs = "auto";
       cores = 0;
 
       builders-use-substitutes = true;
     };
 
+    optimise.automatic = true;
+
     extraOptions = ''
       keep-going = true
     '';
 
     gc = {
       automatic = true;
-      dates = "weekly";
+      interval = {
+        Weekday = 0;
+        Hour = 0;
+        Minute = 0;
+      };
       options = "--delete-older-than 7d";
     };
   };