### Motivation
If we follow a redirect which changes the origin e.g. from `127.0.0.1` to `localhost` we didn't change the `Host` header to the appropriate new origin and port combination.
### Changes
Use the original request which does not include the host instead of the prepared request to form a new request to the redirect URL.
### Alternatives
If the user defines a `Host` header themselves on the original `HTTPClientRequest` we currently never touch it, even in the redirect case. Maybe we should change our strategy and do one of the following:
1. We could always override the user defined `Host` header
2. We could only remove the user defined `Host` header on redirect and set it to the new origin and port combination
### 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, *)`
David Nadoba