Remove OpenStruct from primer_octicon cop

[camertron]

Shamelessly copy/pasting what @HParker wrote in #3052

OpenStruct will not be included in modern Ruby versions, so we should not rely on it being there. This removes usage and instead builds the same class instance that is returned above.

irb(main):003> h = RuboCop::AST::HashNode.new(hash) => s(:hash)
irb(main):004> h.keys
=> []
irb(main):005> h.pairs
=> []
irb(main):006> h.type
=> :hash

[changeset-bot]

🦋 Changeset detected

Latest commit: 1617834

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/view-components	Patch

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