Following ruby-concurrency#840 which allowed all threads created by concurrent-ruby
to be marked as daemon -- specifically in JRuby; the "AtExit" handling
is no longer needed.

Delete all references to "AtExit" and replace it with clearer
documentation that the users can select whether the threads started
shall be marked as daemon threads.

> When a Java Virtual Machine starts up, there is usually a single non-daemon thread (which typically calls the method named main of some designated class).
> The Java Virtual Machine continues to execute threads until either of the following occurs:
>
> 1. The exit method of class Runtime has been called and the security manager has permitted the exit operation to take place.
> 2. All threads that are not daemon threads have died, either by returning from the call to the run method or by throwing an exception that propagates beyond the run method.

- disable_at_exit_handlers! has to be kept as noop not to break current usages
- cleanup auto_terminate handling
- remove ns_auto_terminate attribute
- deprecate auto_terminate=

Add @LikeLakers2 suggestion.

Co-Authored-By: MichiRecRoom <1008889+LikeLakers2@users.noreply.github.com>

Fix grammatical error in configuration.rb

Fix grammatical error in configuration.rb