Refactor(CI Minutes): use customer graphql client

app/assets/javascripts/ensure_data.js

app/assets/javascripts/lib/graphql.js

doc/development/fe_guide/graphql.md

.../javascripts/subscriptions/buy_minutes/components/app.vue

...scripts/subscriptions/buy_minutes/components/checkout.vue

.../buy_minutes/components/checkout/subscription_details.vue

...assets/javascripts/subscriptions/buy_minutes/constants.js

...p/assets/javascripts/subscriptions/buy_minutes/graphql.js

ee/app/assets/javascripts/subscriptions/buy_minutes/index.js

ee/app/assets/javascripts/subscriptions/buy_minutes/utils.js

...ubscriptions/graphql/queries/plans.customer.query.graphql

...scripts/subscriptions/graphql/queries/state.query.graphql

...frontend/subscriptions/buy_minutes/components/app_spec.js

..._minutes/components/checkout/subscription_details_spec.js

...end/subscriptions/buy_minutes/components/checkout_spec.js

ee/spec/frontend/subscriptions/buy_minutes/index_spec.js

ee/spec/frontend/subscriptions/buy_minutes/mock_data.js

ee/spec/frontend/subscriptions/buy_minutes/spec_helper.js

ee/spec/frontend/subscriptions/buy_minutes/utils_spec.js

...ib/gitlab/graphql/queries/plans.customer.mutation.graphql