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

@@ -58,6 +58,8 @@
   add `vimPlugins.notmuch-vim` to your (Neo)vim configuration if you want the
   vim plugin.
 
+- `services.avahi.ipv6` now defaults to true.
+
 <!-- To avoid merge conflicts, consider adding your item at an arbitrary place in the list instead. -->
 
 ## Other Notable Changes {#sec-release-25.05-notable-changes}

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.";
     };