Fetch recent boards using GraphQL endpoint

Related to #300985 (closed) (backend).

Replace the axios call for recent boards in app/assets/javascripts/boards/components/boards_selector.vue using the following GraphQL query (more details in the issue linked above).

query fetchRecentProjectBoards {
  project(fullPath: "gitlab-org/gitlab") {
    recentIssueBoards {
      edges {
        node {
          name
        }
      }
    }
  }
}