Use new mergeability field in merge request widget

When the backend is complete (#367891 (closed)) we need to update the various parts of the widget, most notably https://gitlab.com/gitlab-org/gitlab/-/blob/12a280b7334abb0f98c1b5af2448a34e646bf974/app/assets/javascripts/vue_merge_request_widget/stores/get_state_key.js, to use this new field.

/cc @andr3

Availability & Testing

The user workflow should remain unchanged from this issue but since it is a common workflow in the E2E tests it is recommended to trigger the package and qa job. @jay_mccure to verify results.

Raised #371575 to improve the E2E tests by leveraging the mergeability field.

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