Experimental opentelemetry output

What?

In k6 v0.53, the open telemetry output becomes part of the core, so we're adjusting documentation accordingly.

[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.
[x] I have made my changes in the docs/sources/next folder of the documentation.