keepalived: use `ints.between`

Signed-off-by: Arthur Gautier <[e-mail]>

nixos/modules/services/networking/keepalived/vrrp-instance-options.nix

···
     };
 
     virtualRouterId = mkOption {
-      type = types.int;
+      type = types.ints.between 1 255;
       description = lib.mdDoc ''
         Arbitrary unique number 1..255. Used to differentiate multiple instances
         of vrrpd running on the same NIC (and hence same socket).