New Rule S6966: Awaitable method should be used

[zsolt-kolbay-sonarsource]

RSPEC: SonarSource/rspec#3854

[jilles-sg]

This rule may be useful to complete the final parts of an migration to async or to help prevent incorrect sync calls being added. However, there are cases where a normally asynchronous (I/O) type is synchronous, such as MemoryStream, StringReader, StringWriter or a StreamWriter that writes to a MemoryStream. In such cases, the asynchronous calls are unnecessary overhead. This is particularly nasty when await using has to be used with .ConfigureAwait(false), since a separate variable is required to store the ConfiguredAsyncDisposable.