Fix iterations_in_advance mismatch between backend and frontend

The number of iterations that can be created in advance tops at 10 in the backend:

# ee/app/models/iterations/cadence.rb

validates :iterations_in_advance, inclusion: { in: 0..10 }, allow_nil: true

However, in the frontend, a user is able to choose 12 as an option:

// ee/app/assets/javascripts/iterations/components/iteration_cadence_form.vue

  availableFutureIterations: [
    { value: 0, text: i18n.futureIterations.placeholder },
    2,
    4,
    6,
    8,
    10,
    12,
  ],

The discrepancy causes an error when creating a cadence with iterationsInAdvance=12 (#348763).

Screen_Recording_2021-12-17_at_10.22.42_AM

Fix

Although the backend fix is trivial (changing 0..10 to 0..12 in cadence.rb#20 should do) , we should consider additional changes to ensure the frontend and backend always use a SSOT.

1. Validate the values for the argument IterationsInAdvance through the GraphQL after declaring the possible range in the Cadence model.

2. Possibly use the GraphQL introspection or pass the range (1 to 12) through HAML to the frontend.