Mark Prometheus client as an official extension

[andrewslotin]

What?

xk6-client-prometheus-remote is officially maintained by Grafana team. This PR updates the JSON data file to reflect this.

Checklist

• [x] I have used a meaningful title for the PR.
• [x] I have described the changes I've made in the "What?" section above.
• [x] I have performed a self-review of my changes.
• [x] I have run the npm start command locally and verified that the changes look good.

Related PR(s)/Issue(s)

As requestied in https://github.com/grafana/awesome-k6/pull/28#discussion_r1699644851