Add mod and moderator to list of default reserved usernames (#9713)

config/settings.yml

-# config/app.yml for rails-settings-cached
-#
-# This file contains default values, and does not need to be edited
-# when configuring an instance.  These settings may be changed by an
-# Administrator using the Web UI.
-#
-# For more information, see docs/Running-Mastodon/Administration-guide.md
-#
+# This file contains default values, and does not need to be edited. All
+# important settings can be changed from the admin interface.
+
 defaults: &defaults
   site_title: Mastodon
   site_short_description: ''
@@ -54,11 +49,14 @@ defaults: &defaults
     - root
     - webmaster
     - administrator
+    - mod
+    - moderator
   disallowed_hashtags: # space separated string or list of hashtags without the hash
   bootstrap_timeline_accounts: ''
   activity_api_enabled: true
   peers_api_enabled: true
   show_known_fediverse_at_about_page: true
+
 development:
   <<: *defaults