Use GraphQL for the vulnerability state dropdown

When viewing an individual vulnerability on the security dashboard, there is a dropdown that lets the user change the status of the vulnerability:

Currently, changing the status will call a REST endpoint. We need to use GraphQL instead.

See Design Issue and parent Epic for details.

Implementation Plan

• Change changeVulnerabilityState() and refreshVulnerability() in ee/app/assets/javascripts/vulnerabilities/components/header.vue to use GraphQL. refreshVulnerability() is called from ee/app/assets/javascripts/vulnerabilities/components/vulnerability.vue, we should look into whether we still need it after switching to GraphQL.