Migrate CI to use GitHub Actions.
### Motivation:
To migrate to GitHub actions and centralised infrastructure.
### Modifications:
Changes of note:
* Adopt swift-format using rules from SwiftNIO.
* Remove scripts and docker files which are no longer needed.
* Disabled warnings-as-errors on Swift 6.0 CI pipelines for now.
### Result:
Feature parity with old CI.
## Motivation
Some of the test code was missing availability guards for Apple platforms, resulting in build failures for these platforms, e.g.
```
error: 'AsyncSequence' is only available in iOS 13.0 or newer
```
## Modifications
Add missing availability guards. I've tried to keep them as scoped as possible.
## Result
Tests can now build for run on iOS and other Apple platforms.
* save progress
* Replace `TransactionBody` with `NIOAsyncSequenceProducer`
* test
* revert unnesscary changes
* Add end-to-end test
which currently fails because of https://github.com/apple/swift-nio/issues/2398
* soundness
* Use latest swift-nio release
* throw CancellationError on task cancelation
* Fix Swift 5.5 & 5.6
### 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, *)`
Rick Newton-RogersandGitHub
Si BeaumontandGitHub
David NadobaandGitHub
Fabian FettandGitHub