Rubocop fix: Perfomance/UnfreezeString (#26217)

app/validators/status_length_validator.rb

@@ -45,7 +45,7 @@ class StatusLengthValidator < ActiveModel::Validator
 
   def rewrite_entities(str, entities)
     entities.sort_by! { |entity| entity[:indices].first }
-    result = ''.dup
+    result = +''
 
     last_index = entities.reduce(0) do |index, entity|
       result << str[index...entity[:indices].first]