Remove option to refer to queue selector as experimental

In #147 (closed), we changed the --experimental-queue-selector option to be just --queue-selector, but we support either. In this issue, we can remove the old version. To do so, we'll need to grep for this issue (https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/646) in:

1. The Rails app.
2. Omnibus.
3. Charts.
4. Our CNG images.

And remove / simplify as appropriate.

We also need to mention this in the monthly release post.