Add `hidden` field to GraphQL `Issue` type

📖 What does this MR do?

Related to #332408 (closed)

In https://gitlab.com/gitlab-org/gitlab/-/issues/327353 we added the ability to ban a user. In #327355 (closed) we extended the ban functionality so issues created by banned users are hidden. This MR adds the hidden field to the GraphQL Issue type so we can display a hidden issue icon on boards. The icon will be implemented on the frontend in !69558 (merged) to avoid any potential backwards compatibility issues.

🚦 Does this MR meet the acceptance criteria?

Conformity

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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
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• [-] This change is backwards compatible across updates, or this does not apply.

Availability and Testing

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