modules/avahi: Enable IPv6 by default

Avahi's default for `use-ipv6` is yes as well. I see no reason why we
should do this differently.

nixos/doc/manual/release-notes/rl-2505.section.md

@@ -77,6 +77,8 @@
 
 - Cinnamon has been updated to 6.4.
 
+- `services.avahi.ipv6` now defaults to true.
+
 - `bind.cacheNetworks` now only controls access for recursive queries, where it previously controlled access for all queries.
 
 <!-- To avoid merge conflicts, consider adding your item at an arbitrary place in the list instead. -->

nixos/modules/services/networking/avahi-daemon.nix

@@ -91,7 +91,7 @@ in
 
     ipv6 = lib.mkOption {
       type = lib.types.bool;
-      default = false;
+      default = true;
       description = "Whether to use IPv6.";
     };