Migrate `app/assets/javascripts/contributors/stores/index.js` to Pinia
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[`app/assets/javascripts/contributors/stores/index.js`](https://gitlab.com/gitlab-org/gitlab/blob/3e60bba206715737ac53d6e50fea4b76a425b6d6/app/assets/javascripts/contributors/stores/index.js) store should be migrated to Pinia using [the single-step migration](https://docs.gitlab.com/development/fe_guide/pinia/#single-step-migration).
Files to cover (non-exhaustive):
- `app/assets/javascripts/contributors/stores/index.js`
- `app/assets/javascripts/contributors/stores/actions.js`
- `app/assets/javascripts/contributors/stores/getters.js`
- `app/assets/javascripts/contributors/stores/mutations.js`
- `app/assets/javascripts/contributors/stores/state.js`
- `app/assets/javascripts/contributors/index.js`
- `app/assets/javascripts/contributors/components/contributors.vue`
And their respective specs.
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