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
Fixes#238 and #231.
### Changes
- Extracted the unclean shutdown test from `HTTPClientTests` into their own file `HTTPClientUncleanSSLConnectionShutdownTests`
- Copy and pasted @weissi great explanation from #238 into the test file
- Removed property `ignoreUncleanSSLShutdown` everywhere
### Result
`ignoreUncleanSSLShutdown` on `HTTPClient.Configuration` is deprecated and ignored.
Co-authored-by: Johannes Weiss <johannesweiss@apple.com>