Rubocop fix: Perfomance/UnfreezeString (#26217)

.rubocop_todo.yml

@@ -158,14 +158,6 @@ Naming/VariableNumber:
     - 'spec/models/domain_block_spec.rb'
     - 'spec/models/user_spec.rb'
 
-# This cop supports unsafe autocorrection (--autocorrect-all).
-Performance/UnfreezeString:
-  Exclude:
-    - 'app/lib/rss/builder.rb'
-    - 'app/lib/text_formatter.rb'
-    - 'app/validators/status_length_validator.rb'
-    - 'lib/tasks/mastodon.rake'
-
 RSpec/AnyInstance:
   Exclude:
     - 'spec/controllers/activitypub/inboxes_controller_spec.rb'