Fix fetch iteration cadence by id

What does this MR do?

This is being behind FF iteration_cadences. To try it out make sure to enable the FF either globally or for a specific group

Feature.enable(:iteration_cadences)

Fix iteration cadence fetching by ID. There is no related issue for this but I did find it fails by testing out iteration cadences management at https://gitlab.com/groups/gitlab-org/plan-stage/-/cadences/491/edit

GraphQL query:

query($id: ID!){
  project(fullPath: "top-group/sub-group/test-empty-project"){
    iterationCadences(id: $id){      
      nodes{
        id
      }
    }
  }
}

// id: as integer
{ "id": 13547 }

// id: as global id
{ "id": "gid://gitlab/Iterations::Cadence/13547" }

Screen_Recording_2021-09-02_at_13.58.19

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How to setup and validate locally (strongly suggested)

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