wireguard: remove dependency on ip-up.target

It was deprecated and removed from all modules in the tree by #18319.

The wireguard module PR (#17933) was still in the review at the time and
the deprecated usage managed to slip inside.

nixos/modules/services/networking/wireguard.nix

@@ -151,7 +151,8 @@ let
     nameValuePair "wireguard-${name}"
       {
         description = "WireGuard Tunnel - ${name}";
-        wantedBy = [ "ip-up.target" ];
+        after = [ "network.target" ];
+        wantedBy = [ "multi-user.target" ];
         serviceConfig = {
           Type = "oneshot";
           RemainAfterExit = true;