nixos: system: printing: migrate deprecated option

It's recommended to only enable the IPv4 option, as most mDNS responders
only register IPv4 addresses (therefore enabling IPv6 would lead to long
timeouts when checking for those addresses first).

modules/nixos/system/printing/default.nix

@@ -63,7 +63,7 @@ in
       enable = true;
       openFirewall = true;
       # Allow resolution of '.local' addresses
-      nssmdns = true;
+      nssmdns4 = true;
     };
   };
 }