Proposal: rename `async def close()` to `async def aclose()` across aiohttp

### Describe the solution you'd like

`contextlib.aclosing()` expects the object to provide an `aclose()` coroutine, which it awaits automatically. This makes `aclose` the de-facto naming convention for asynchronous close methods.
For consistency with this convention, I propose renaming `async def close()` to `async def aclose()` on all aiohttp classes that currently expose an async `close()`. Backward compatibility should be preserved: the old `close()` should keep working but emit a `DeprecationWarning` when called.

### Related component

Server and Client

### Code of Conduct

- [x] I agree to follow the aio-libs Code of Conduct