Add Style/OpenStructUse Cop

[mttkay]

This flags any use of the OpenStruct type, whose use is now officially discouraged.

Originally written as part of https://gitlab.com/gitlab-org/gitlab-styles/-/merge_requests/92.

This PR addresses existing OpenStruct offenses in RuboCop itself as well.

Fixes #10206

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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.

[mttkay]

Tests are failing because RuboCop uses OpenStruct 😬 Where's the dreidel emoji.

[dvandersluis]

Can you add a test that no offense is registered for MyNamespace::OpenStruct.new?

[mttkay]

Thanks for all the thorough feedback @dvandersluis ; before I get into cleaning up the commit history, would you mind having a final pass over the changes?

[dvandersluis]

@mttkay this is looking great! I added one last comment but this is basically good to go once squashed. Thanks for your work! ❤️

[mttkay]

Thanks for all the help @dvandersluis -- I believe this is ready for a final review. I added a changelog entry and squashed commits.

I noticed that the spellcheck jobs require additional approval?

This workflow is awaiting approval from a maintainer in #10217

[dvandersluis]

Thank you again for your work on this! It's looking ready to go for me now.

I noticed that the spellcheck jobs require additional approval?

Github actions runs have to be approved for first time contributors, it's running now.