upgrade: Add anvil socket changes

Author: sirainen

docs/installation/upgrade/2.4-to-2.4.x.md

@@ -37,3 +37,5 @@ changed to `2.4.2`.
 | Setting | Old Default Value | New Default Value |
 | ------- | ----------------- | ----------------- |
 | [[setting,mail_attachment_detection_options]] |  | `add-flags content-type=!application/signature` | |
+| `service/anvil/unix_listener/anvil/mode` | `0600` | `0660` |
+| `service/anvil/unix_listener/anvil/group` | (empty = root) | `$SET:default_internal_group` |