Enable RuboCop rule `Performance/OpenStruct`

What does this MR do?

Follow-up of !55899 (merged).

Exclude current offenses in .rubocop_manual_todo.yml and link the follow-up issue.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • [-] I have included a changelog entry.
  • I have not included a changelog entry because it's developer-facing change.
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content