FIX: Minimum username length should be validated (#31332)

`@min_length_violation` was not defined and that made the range of
values error message to never be displayed.

lib/validators/min_username_length_validator.rb

@@ -18,7 +18,7 @@ class MinUsernameLengthValidator
   end
 
   def error_message
-    if @min_length_violation
+    if @min_range_violation
       I18n.t(
         "site_settings.errors.invalid_integer_min_max",
         min: MIN_USERNAME_LENGTH_RANGE.begin,