update queue_classic details

This includes updated details about queue_classic in ActiveJob. I am one of the maintainers of queue_classic.

- the original creator always referred the project as "queue_classic", not Queue Classic (I made that mistake more than once too)
- the repo has moved to a "QueueClassic" org (I know, this feels inconsistent, but it was not possible to get "queue_classic" for the org name...so...)
- I tried current queue_classic stable (3.x) last week with ActiveJob, and it works, so I removed the "2.x" from the doc here.

Also, there is some work being done to add job scheduling and I am wondering if it should be added to the note already included in `queue_adapters.rb`?

Thanks!

/cc @senny