### Motivation
With Xcode 13.2, and therefore Swift 5.5.2, Swift Concurrecy is supported on older Apple OSs. async/await suport will no longer be available on Swift before `5.5.2` but this isn't a breaking change because we have not yet made anything of it public.
### Changes
- replace all `#if compiler(>=5.5) && canImport(_Concurrency)` with `#if compiler(>=5.5.2) && canImport(_Concurrency)`
- replace all `available(macOS 12.0, iOS 15.0, watchOS 8.0, tvOS 15.0, *)` with `available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)`