Make RuboCop's TodoFormatter entry a class not a struct

What does this MR do and why?

Follow-up of !82100 (comment 865280757).

How to set up and validate locally

bin/rspec spec/tasks/rubocop_rake_spec.rb  spec/rubocop/todo_dir_spec.rb  spec/rubocop/formatter/todo_formatter_spec.rb

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.