Drop the async_ prefix from connection functions

I don't see a reason to keep them anymore. The connection will never provide sync functions to there won't be any confusion.

```cpp
co_await conn.run(cfg);
co_await conn.exec(req, resp);
co_await conn.receive();
```

Look cleaner too.

No file or test is named. Start by locating the connection API and all references to the async_ connection functions, including run, exec, and receive; inspect nearby tests or examples. Done means the connection API and its references consistently use the proposed names without ambiguity.

Written by the indexing model from the issue text.