Rename the #async channel

[swfarnsworth]

We seek to name topical channels in such a way that it's easy to tell at a glance what might be on-topic there, even if the name that we select doesn't formally capture everything. For example, we renamed "python-extensions" to "c-extensions", even though compiled Python modules written in any language is on-topic there. Likewise, we renamed "automated-testing" to "unit-testing", even though other types of automated tests are on-topic, since "unit testing" is often used informally to describe any level of automated test. Finally, we recently renamed "data-science" to "data-science-and-ai" to make it more obvious to all our users that AI (and machine learning) are on-topic there, since newcomers to data science may not realize that "data science" is often understood to encompass AI and machine learning as well.

In that light, I'd like suggest that we rename "async" to "async-and-concurrency" so that it's more obvious that concurrent code that doesn't necessarily use the async and await keywords is on-topic. While the channel description gives other topics, I know that when I first saw the channel, I read "async" and concluded that multiprocessing is off-topic for that channel and that no channel in which multiprocessing is on-topic existed.

While the channel description clarifies what is on-topic, it is not likely that anyone will check if in their mind, the headline and their desired topic are unrelated.

For reference, the current channel description is as follows:

Discussion about concurrency in Python. Asyncio, trio, twisted, multiprocessing, threading, the GIL, or other related topics.

[swfarnsworth]

This change was made after we discussed it during our staff meeting today.