Workaround for Ruby's warning in YARD

[koic]

This is a workaround to prevent the following warning in YARD:

$ ruby -v
ruby 3.4.0dev (2024-04-19T07:33:16Z master 7951b349ab) [x86_64-darwin23]
$ cd path/to/rubocop
$ bundle exec rake
/Users/koic/.rbenv/versions/3.4-dev/lib/ruby/gems/3.4.0+0/gems/yard-0.9.36/lib/yard/templates/template_options.rb:2:
warning: ostruct was loaded from the standard library, but will no longer be part of the default gems since Ruby 3.5.0.
Add ostruct to your Gemfile or gemspec.

The fundamental resolution will likely be addressed in lsegal/yard#1545.

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.