Since #23152 eliminated the EventMachine dependency, we don’t need to
explicitly mention EventMachine.

Nevertheless, I'm not 100% sure about saying "the websocket-driver loop"
driver… any suggestions, @matthewd or @pixeltrix ? 😅

[ci skip]

We're no longer doing our work in the EM event loop, so errors are quite
unlikely, and if they do occur, they're not really our responsibility to
handle.

(as adapted to use concurrent-ruby / nio4r instead of eventmachine)

Just like their ActiveJob equivalents, these only work within the
current process.

This changed in #22950.

I believe the paragraph starting with "This relies" belongs with the one above it.

Fixes #23193

This was throwing `There was an exception - NameError(undefined local variable or method `hi_redis_conn' for #<ActionCable::SubscriptionAdapter::Redis:0x007fb1449e2b70>)` on unsubscribe.

[ci skip]

- adapter -> pubsub (re)rename internally
- Change variable names to match method names
- Add EventMachine `~> 1.0` as a runtime dependency of ActionCable
- Refactor dependency loading for adapters

- Escape the channel name when subscribing in PG
- Refactor popping the queue to make it easier to read

We now only create one adapter instance for the server, so it can hold
the listener. This in turn allows the listener to get the PG connection
from the adapter, which will be a good place to allow more flexible
configuration.

* Clarify wording around how defaults work
* Clarify wording around what formatting versus information is required
* Separate consumer configuration section since it is important and wordy
* Add additional explanation of options for setting consumer configuration
* Consolidate consumer configuration information to reduce confusion

This reverts commit d0393fcc, reversing
changes made to 3b7ccadf.

Found this small omission while reviewing the ActionCable docs.
[Dave Moore]

This removes 8 runtime gem dependencies from Rails:

```
Using hitimes 1.2.3
Using timers 4.1.1
Using celluloid-essentials 0.20.5
Using celluloid-extras 0.20.5
Using celluloid-fsm 0.20.5
Using celluloid-pool 0.20.5
Using celluloid-supervision 0.20.5
Using celluloid 0.17.2
```