Add milestone_id to issue update graphQL mutation

What does this MR do?

It adds milestone_id to the issue update mutation.

Examples

Setting milestone

mutation {
  updateIssue(input: {projectPath: "flightjs/flight", iid: "607", milestoneId: 54}) {
    issue {
      id
      milestone {
        id
        title
      }
    }
    errors
  }
}

Removing milestone

mutation {
  updateIssue(input: {projectPath: "flightjs/flight", iid: "607", milestoneId: null}) {
    issue {
      id
      milestone {
        id
        title
      }
    }
    errors
  }
}

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Issue #36312 (closed)