Consistent naming for methods that return Tasks

[lukebakken]

https://github.com/rabbitmq/rabbitmq-dotnet-client/pull/1264/files#r1096793817

https://stackoverflow.com/questions/15951774/does-the-use-of-the-async-suffix-in-a-method-name-depend-on-whether-the-async

https://stackoverflow.com/a/15951824

The Task-based Asynchronous Pattern (TAP) dictates that methods should always return a Task (or Task) and be named with an Async suffix; this is separate from the use of async.

[lukebakken]

Addressed by #1347 and future 7.0 PRs