Mark `CompletionStage<T>.await()` as `@DelicateCoroutinesApi`

[dovchinnikov]

This is unexpected that await cancels the CompletableFuture on coroutine cancellation. Since it's named like Deferred.await, it's expected to behave the same way without requiring to convert to Deferred first.

[dovchinnikov]

This could be already done before the decision on #4329.

[dkhalanskyjb]

I'd prefer to decide on #4329 in the near future. If we decide to deprecate CompletionStage.await anyway, then forcing everyone into an opt-in first is going to create unnecessary friction.