Fix update/create board mutation for iteration_cadence_id param

What does this MR do and why?

We were not handling iteration_cadence_id GlobalID properly on both create and update mutations for Boards.

Related to #323653 (closed)

How to set up and validate locally

Try to update a board's iteration cadence through GraphQL with a query like

mutation {
  updateBoard(input: {id: "gid://gitlab/Board/615400", iterationCadenceId: "gid://gitlab/Iterations::Cadence/10391"}) {
    board {
      iterationCadence {
        id
      }
    }
  }
}

Iteration cadence should not be updated before this MR.

THe same happens for board create with a query like

mutation {
  createBoard(input: {groupPath: "gitlab-org", name: "Mario's Test Board", iterationCadenceId: "gid://gitlab/Iterations::Cadence/10391"}) {
    errors
    board {
      id
      name
      iterationCadence { id }
    }
  }
}

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